Carrot:

I use plastic bags designed to transport fish from the pet store. They'll give you a few for free if you ask nice. They fold up small and are tough, but have no closure -- zip lock etc. (That's one reason why they fold up so small.)

I went one step further and ordered a case of 1,000 from a manufacturer. I'll send you a dozen or so if you send me a self-addressed, stamped envelope. E-mail me or PM me for the mailing address.

Bear