I was thinking more towards AW girls who provide these services rather than professional dommes.
I would suggest that is a risky approach for a newbie to pegging. Lots of AW girls list/provide strap-on services, but that doesn't mean they know what they are doing. You could end up with an woman not doing enough warming up and then ramming a strap-on up your arse. Which understandably would put you off for life.
You would be better off, and less sore, paying the extra money and finding an experienced domme who specifically mentions beginners in her profile and will introduce you slowly and gently (with plenty of lube) and then use a suitable sized and agreed strap-on.
Practice at home is also advised, using an anal dildo and lube. And, learning how to douche properly.