I have that thought too....where the people smaller than me shop?? I've lost a lot and also frequent the kids section but every day I see girls far skinnier than me and have no idea where they get their things.
The little girls' L/XL is like an adult XS/S just cheaper so if it fits it's worth it.
H&M and Forever 21 are good price-wise but their sizes are very inconsistent.
If you have a Kohl's in your area they have a decent junior dept and lately I've been loving Abercrombie kids...same clothing, about $20 cheaper, and they don't douse the store with cologne like the adult store does.
And I'll also recommend this ebay store, it's a one-size fits all from Hong Kong so really it's one size fits all small but they have cute things, very cheap, shipping is a little high but it equals out with the low price.
http://stores.ebay.com/adaskoreafashionstore