D.K.
You can also try Kohl's for good prices. I agree probably a good idea to have her measured if she is okay with that. Have her try on some different ones- sport bra type, underwire, no underwire to see what is comfortable. Also different brands. The sales racks a lot of times will have small cup sizes on sale. Some will also have AA that can be great, I had a few of those when I was a teenager, but can't remember the brand. One brand, not sure which, has inbetween sizes now too, so they have an almost A. I love the brand barely there and they aren't very expensive either. I have worn several of their different ones, used to be the minimal support, no clasp, no wires now I wear one with a little more support and a catch, but it is still really comfy for all day.