As far as advise, I would definitely show up the the clinics in person. Have a nice letter that explains your program and what your objective is ( to find a quality preceptor). And as stated above, offer to financially compensate them for their time and experience. They deserve that and of course the schools should be there ones to be paying them. The times we are in are extremely competitive. Markets everywhere have a plethora of NP students. In my area, the NP jobs are few and far between. That, and many are offering less money than I make now as an RN. The market here is one of the worst from what I see.
I wouldn't give up until you have exhausted all your options and tried everything.