I guess baby formula might do, but I really don't think it's best. Baby formula and kitten formula are different. Just like you shouldn't give can't regular milk you'd drink...the kitten should have KMR. I can't believe the vet wouldn't have that or know of a good alternative. If she lives in a rural area, are there any smaller petstores closer?...also try feed stores, they should have KMR to.
Good Luck.