Fetishes today are no longer taboo. In the last 5-10 years or so in p4p, many gals have incorporated role play, S&M, B&D, etc scenarios to their menus. They don't have to be domme only girls like back in the day.
If eros is in your area, they have a section for S&M so obviously the girls in that section won't have an issue with you asking about fetish. P411 is also specific in their ads as to what girls will do in session, in many cases.
But like I said, since it has become more mainstream, don't rule out girls that don't advertise in an S&M section. If you see a hot girl you are attracted to, scope out her website and see if it says "fetish friendly" anywhere or if her pics suggest she is open to the possibility. (She is wearing leather/latex, she's pictured with toys, etc). Some girls don't advertise their S&M services for they feel it may hurt their escorting business but that is few and far between these days.
Make sure they have reviews here too. Read thru this Newbie Board for tips. You need to protect yourself against LE (cops), rip off artists and bait and switch types. TER is invaluable in this area.
With any girl you contact, just be very polite and ask her upfront if she offers "fetish", for the girls that don't mention it on their site.
If she says yes, she may ask you what fetish you are referring to. If she simply replies with a "yes" only, follow up and ask if you can discuss your specific request.
Be cautious though. Don't mention any sex acts, especially before she verifies you. After you have been screened and given the ok, she may be open to discuss specific sex acts but let her open that door.
Get back with any questions. Good luck JW.