If you are looking to switch vets, it's probably easiest to start with Google or your online Yellow Pages and call around. Ask the receptionist or vet assistant at each practice to call you back when they have a few minutes to chat. You can tell a lot about a practice by how professional and understanding the staff is. Ask if any of their vets specialize in GI disorders, and where they refer difficult GI cases. When you find a GI specialist, see if she/he can schedule a few minutes to call and chat with you, too.
You might begin with all-cat practices, if there are any near you. They tend to have seen everything, and more of it, and they should keeping up-to-date on feline medicine.
If you have a choice, take a practice that offers 24-hour emergency service over one that doesn't. There's nothing scarier than racing a very sick cat to a strange vet in the middle of the night.
When I found out that my kittens had ringworm, I called vets all over the city to find out how they treated it and whether they would do the medicated dips all my cats needed. I talked to various helpful or clueless assistants and spoke with some vets. I learned a lot, and decided to stick with my own vet's plan. But I now know where I'm going if I ever do need to switch vets.
Good luck!
PS: If you are near Bethlehem, PA, I had a good experience with Wright's Veterinary Hospital. It wasn't a GI issue but they managed to save an ancient cat with advanced injection-site sarcoma — there's one darned good surgeon there.
