Walmart carries some nice ones for about $5. They are expandable mini files type. I don’t use envelopes I have a small wallet with several pockets in it and I’ve labeled the pockets for the right “envelope” and then store the proper loyalty cards in that pocket of the wallet along with the cash. That way it’s all together in one spot.
Also when looking at office supply stores look at the Martha Stewart line of organizers, some of them are not girly and they are pretty sturdy.
Craft stores like Hobby Lobby carry “envelopes” of different materials as well.