Don't be too sure about not growing anymore at 20! I increased FOUR CUP SIZES between the ages of 19-25! I went from nice and average, to breast reduction surgery at 27.

As far as small sizes...lots of stores carry them, even stores like walmart, target and kmart. One other thing to be aware of regarding children's departments (aside from the H.M. and Barbie issue)...little girls are not just small cup sizes, they are also small around the rib cage, which is an important part of the bra size. If you can't wear a girls size shirt easily, then you probably can't wear a girls sized bra either, no matter how small your cup size is.