Well, if you were in Harlow I could take you to a shop where you get ten of them for £1 - tough A5 envelopes with some built-in bubble wrap. The CDs always seem to get through undamaged

That's not much help really, since you aren't in Harlow. But I'd be surprised if there aren't equivalent shops in the USA - it's the sort where they sell everything for the same price, £1, and the bargains come and go. But they always seem to have the envelopes.

Otherwise maybe you could get hold of tough envelopes in some discount store, and just stuff some bubble wrap inside.