We've started using really cute messenger book bags with our letters embroidered on them as the girls' bid bags. That way they have a new book bag to start school off with and its a great marketing tool!
I have no idea where we order them from, but I'm pretty sure you can get them made just about anywhere....from ordering online to calling your nearby greek store.
We also like to include Alpha Phi car decals in the bags. We get those at the greek store.