While I generally agree with Laserface when he says "most providers who "claim" to accommodate fantasy requests actually don't." and most who quickly say sure are not good at it
I will disagree when he says "It's very unlikely that you'll be able to get a provider you've never seen before to do a session like you're describing in any case". I believe it is possible to find a provider who can accommodate your fantasy request BUT you must do a lot of research on them upfront.
Laserface is correct in that if you just approach random ones or ask haphazardly, you will fail. But over the years, I have seen 3 providers that were not only willing to accommodate some fantasy roleplay on my initial visit with them - but were damn good at it and as a result were some of my more memorable sessions. Now what was it that these three had in common - they all appeared intelligent, were all in their mid 30's - mid 40's, were not hoodrats, charged a bit more than my typical budget, had either reviews that led me to believe they were open to what I had in mind or mentioned in depth on their website some of the different things they were into (which even if not the same led me to believe they would be able to pull it off well).
So I would suggest doing your research and once you find someone who you think may be a good probability, reach out to her and start simple like Laserface suggested when he concluded with this which is very sound advice
One of my "touchstone" rules for things like this is that if I ask a provider if she'd be open to consider a fantasy request, and the answer is anything resembling, "Umm... Well, I guess it depends on what you're interested in...", I automatically give up and move on. Because (again, in my experience) that kind of response is basically a polite way of trying to tell you, "I'm really not comfortable with this, but if it's simple, I'm willing to try." If I ask this question and get anything other than an enthusiastic "yes", at least for the general concept of fantasy play, I can be almost positive that even if I can get her to try, I'm going to be disappointed.
YMMV, of course.
Originally Posted by laserface
If you are smart, you will know right away whether to move on or engage further. As for R&D time - no idea what you have in mind but I never had a problem once I found a likely candidate asking if it was ok to send them a scenario (warning them it read more like a page long short story not just one or two sentences). And there was never any charge for "R&D". But YMMV