First, thank you for the question. There is so much silence and shame around sex and aging, as well as sexuality and any kind of illness or disease, that many people experience a loss like the one you're describing, but never feel like they can reach out to talk about it or ask for support. Asking for help is hard for most of us, and I appreciate you being able to ask.
As for guidance, I'll do my best but want to acknowledge that so much depends on you and your wife, as a couple and as individuals.
In terms of your wife, the way people who have been diagnosed with Alzheimer's interact with others is so varied, and while I want to provide some concrete guidance I need to acknowledge that my suggestions may be totally off base. You and your wife may be still having coherent conversations. Or possibly not. If your wife gets violent and physically aggressive it may not be safe to explore some of these options at all.
You may have already thought of everything I'm going to suggest. I'm aware that you haven't asked about appropriateness, so I don't want to spend much time on that, but I do know that for many partners it can feel as if seeking sexual contact or intimacy elsewhere is too great a betrayal. And yet we all need support and comfort and touch, and the slow loss that one experiences when a partner develops dementia can itself seem like too much to bear, making our need for intimate contact with someone even more pressing. It's impossible for me to know what your experience of this is, but I just wanted to offer that it is your right and responsibility to take care of yourself, and if you are responsible for taking care of your wife as well, then taking care of yourself is part of that.
I hope you'll bear with me and not be insulted if these ideas don't fit. You're welcome to email me back with more information and I can try and come up with another round of suggestions.
In your email you say that your wife no longer wants any form of sexual contact. What I'd want to know more about is how that has been communicated. Is it something that one or both of you have talked about? Is it that when you try to initiate sexual contact she shows no interest, or actively shows that she isn't interested? Aside from sexual conversations, do you feel as if there are times when the two of you still communicate clearly, in a way that feels like you are both being heard and understood? Or are the moments of clear thinking and communicating from her very rare?
I ask these questions because the silence most of us experience around sexuality throughout our lives becomes a huge obstacle when we experience illness or disease that may lead us to think or communicate differently (as is the case with Alzheimer's). I ask because it's worth thinking about whether you are interpreting her actions accurately. You may assume you know what a response from her means, but if you're able to ask her about it you may find that it means something else. You shouldn't assume that her sexual interests or desires must be the same or must be different as a function of getting older of of Alzheimer's. We are always changing, and the ways we experience and express our sexuality may change dramatically over time (think, for example, of how you expressed your sexuality when you were 14 and how you expressed it when you were 44).
If you feel as if none of the "usual" forms of intimate and sexual contact are acceptable to your wife anymore, one thing to consider is whether there are other forms of contact that would be okay with her and with you. Intercourse might be off the table (for now), but how would she feel about you sitting and combing her hair? Or massaging her shoulders? Or even an everyday kind of action like putting moisturizer on her hands? Some people turn away from touch and human contact, but most people, and most bodies, crave some form of connection. So my first thoughts are to wonder about whether or not it would help to expand what you may be thinking of as intimate or sexual contact.
If what you're craving is sexual contact that involves nakedness and sexual release, that may be possible as well, but it might be that you have to start somewhere that feels lighter or safer. If your wife forgets who you are, and believes you to be a stranger, then it makes sense that she may not want to be sexual with you. On the other hand, she may think you are someone else who she does want to be sexual with, very sexual. Or her attitude toward engaging in sexual activity may have nothing to do with how she is thinking. I know some people who have struggled with this as they aren't sure if it's okay for them to be having sex with their partner when their partner thinks they are someone else. No easy answers here.
Moving on to the second part of what I read in your email, which is that you may want to start being sexual again with other people. I'm not sure what you mean exactly by "best course" but certainly if it isn't already part of your sex life, I want to suggest the best place to start is with yourself, meaning start with masturbation. If you haven't been sexual with someone else in a long time, and if you haven't been sexual with yourself, it's worth taking stock of your sexual response. Our bodies change as we age and also change with life experience. How you feel pleasure and where you feel pleasure may have changed. The good news is that this kind of research is fun. You can explore masturbation just with your own body, or you may want to try out sex toys.
If you're interested in finding someone to have sex with you need to consider the boundaries of what you want and what you're willing to offer someone. Are you looking for a relationship? Just sex? Would you prefer to find someone you can be sexual with who you don't have to talk to about your life, or would you like someone who can be both a sexual and emotional companion? One of the benefits of meeting people online (through dating sites or other venues) is that you're able to be up front about what you want, it's sort of expected. I can recommend both About.com's Guide to Dating and Guide to Senior Living for suggestions in this area.
You mention sex workers as an option, but say that you're concerned about the health implications. I can't recommend you break any laws in the country you live in but I also don't know if you live somewhere where sex work is illegal. So let me share this.
There is a lot of misinformation and stigma about sex work and sex workers, just as there is about aging and old people. It's unlikely that any sex worker would ever object to you asking to use protection that would make whatever kind of sex you want to have much safer. Many sex workers will be much better (and more practiced) at negotiating both personal boundaries and safer sex practices than a casual sexual partner you might find elsewhere. Sex workers, the good ones, are professionals and take their professional obligations seriously. So your health concerns, which are valid, are actually quite easily addressed.
For someone who has a complicated life, with many obligations, a benefit of having sex with a professional sex worker is that they are a professional and will have more experience maintaining boundaries around intimacy and privacy. You need to find a good sex worker, one who is professional and cares about their work. Fortunately there are just as many of those as there are good therapists, dentists, or bus drivers.
Ultimately it's good that you're aware of your sexual needs and desires, and you should know that you have the right to work on getting your needs met, while balancing your obligations for caring for your wife. Whether you choose to pay for sex, find a casual sex partner online, or pursue some other sort of relationship I hope your choice is guided as much as possible by interest and desire and is blocked as little as possible by our general ignorance and shame around aging, sex work, and desire.