Most places willl do it... just do what I do.
When you get to the clinic, try to find out the name of another attendant that is NOT the one working on you.
Then, when you and your attendant are alone and she asks you how you like it, simply tell her, "last time Janet used her hand to finish me off, but she says you're OK with doing oral, so can we try that this time?"
It works even better if you're already naked and have a hard-on.