TheThriftShopper.Com National Thrift Store Directory Listing Charity Resale, Second Hand, and Consignment Shops..You just enter the zip code for where ever you are and suddenly you have the list of all local Thrift Stores..great link
This is something I made for the decor in my house this summer, but it totally works in the classroom. I went to a thrift store nearby and found an old, framed picture (for $4.99!) stuck some adhesive letters on it, and BAM: something pleasant. ("Depaysement" is a French word for the feeling that comes from not being in one's home country. This neat article has a list of other cool words that you could use for an etymology lesson, or just for fun!)
Buying used kids clothes is a SMART way to use your resources. Get more clothes, get better clothes and be less frustrated when stuff gets ruined by kids being kids. Worried you don't have a good thrift store nearby? Have I got a deal for you!