The sweet baby girl in my signature line had trouble on formula, so I suggested to her mother that she use Jarrow Formulas Baby's Jarrow Dophilus. In two of her bottles her mother put 1/4 of a teaspoon of this powder. I can't offer you any advice about formula, outside of what others have said above about staying away from soy formula, but just wanted to suggest that you consider adding in a little acidophilus to whatever formula you decide to go with. Here is a link to the place where I bought it:
Jarrow Formulas Baby's Jarro Dophilus -- 2.5oz (70g) Powder - www.Vitacost.com