I would say Old Navy! Also some of the more trendy shops will have sizes like double zero and things. If you shop online for most stores you will have way more selection in regards to size. If you find something she likes in a store, you should check the website to see if they have more size variety. (I am an extra long in pants, a size that is impossible to find in store but they have online at American Eagle :) )

Forever 21, Warehouse One, H & M, Stitches, Gap, Tommy Hilfigur and others, but those are the ones that I can think of right now.