The girls sizes are listed below those of the guys, but many sell out very quickly (especially at sale time). A good bet is the real-time stock chart that will show you what sizes are available in which prints. You can use the dropdown selector to only list shirts available in your size.
I have bought 16 tees from this site and vouch for the quality. There's a little referral link on the click-through that gives me a buck or so credit on every purchase as a bonus. Also, if you submit a photo of you wearing a shirt to their gallery, you can also get credit of your own for future purchases.
They're not for everyone (not that they're offensive, just made by designers largely for other designers), but pretty unique and a better buy than a major brand name shirt (here, profits go to a small, growing company, and the infamy goes to the small-time designer).