You can find a board certified internist here:
http://www.acvim.org/PetOwners/FindaSpecialist.aspx
Personally, I would start there since there really is nothing diagnosed as yet. Sounds like too much of a guessing game going on right now. I am never comfortable with that. Whenever I go to the specialists they are very thorough and give me a list (usually a short one) of things that could be the problem. They then give an estimate of cost. I have never been dissatisfied.