I love Alloy and Delias. Those are my two favorites, but KarmaLoop and Lucky Brand Jeans (They sell more than jeans) are also great stores.
At fifteen dollar store everything is $15! (Everything!)
Gojane.com has pretty good deals, especially the sales, and you should check the sale sections at Charlotterusse.com, Mandee.com, and wetseal.com.
If you are interested in used clothing, try swapstyle.com where you can swap or buy (very cheap) clothing, often brand name, with other girls around the world. It is a great way to get rid of old clothing that is still in good condition and get "new" clothes.
Also, to get really great deals, try some of the coupon codes below!!
Alloy: $10 off order of $25 or more until 6/30/08 (AHGGIFT)
Delias: $10 off any purchase over $25. Expires 6/30/08 (DWGGift)
Lucky Brand Jeans: FREE Lucky T-Shirt with the purchase of any Lucky Brand Jeans (8FREET)
GoJane: 17% off accessories and swimwear (GJSWIM17)
Get Free Standard Shipping on all US Orders over $75 ("FREESHIP")
Mandee: 20% off entire purchase (JUST4U) or (SUMMER08)
Some cute, cheap, teen clothing stores online are Garage, and Forever 21. Forever 21 is pretty popular, but you didn't mention it so I just said it. Both stores are so cute and cheap! You can buy comfy and soft stuff from those stores. Both their clothes run kind of big so they should fit you. You didn't say American Eagle, but you probably know it but if you don't really shop there, you might want to check it out but some of their stuff is a bit expensive.
If you want to see American Eagle, here is is(their clothes run pretty loose):
Hope this helps :)