There have been TONS of studies on this subject so it may be worthwhile to read some and see if there are gaps or areas that haven't been addressed that would make your work unique.
I would contact local therapeutic riding centers, pediatricians, school nurse or counselor, local autism chapter, and some physical/speech/and occupational therapist for starters. Always be sure to ask if there's someone else they could recommend. Be sure to ask about not only success stories, but cases where equine assisted therapy did not work.
I think if you read what's out there then do your interviews without a preconceived outcome, you'll be surprised at the direction of your final effort. Good luck!