DS is 3 months old and we are switching to formula, he's already taking 4-5 oz so I know we soon will need bigger bottles. We've just been using the 5 oz medela that came with my pump. For my daughter she stayed on BM till one and never needed bigger bottles.

Did you use 8 oz medela bottles? Or did you switch to a different kind at that point? They seem expensive for what they are!