What are you doing to the shirts? I have done the reverse tie-dye with a dark color shirt and bleach. When I do that I like 100% cotton and I find them at our local Walmart in the mens dept for around 5 bucks. I think they are Hanes or Fruit of the Loom.
Started out buying shirts at the local Michaels or AC Moore craft store. Several years ago, I discovered blankshirts.com and have never gone back to the local stores for shirts. My favorite brand is Gildan, preferably one of the cotton/poly blends to avoid shrinkage. Great sweatshirts, too!