You might think that a well-educated, theoretically sophisticated person like me (back off, folks, I said “theoretically”!) might feel a little silly carrying money around in an envelope. I don’t, though, because I learned to do this by following some basic, commonsense financial advice from a guy named Dave Ramsey. He advocates using envelopes with budgeted amounts as a way of keeping a eye on expenses.
I carry just one envelope (groceries/household/food funds), and formerly used Dave Ramsey’s own envelopes. Unlike the office supply variety, Ramsey’s are just the right size for currency, making them convenient to carry around. But they’re paper, and mine tended to get a little ragged, so I laminated them with contact paper. Then I duct-taped the edges, like this:

Ugly, no? When I couldn’t take it any more, I hauled out some iron-on interfacing, a thrilling tropical print, and set to work.
First, I opened out one of the original paper envelopes and traced it on the interfacing. Then I cut out a corresponding piece of fabric, adding an allowance for the side seams, around the fold-over flap, and along the top edge. Then I applied the interfacing to the fabric.

This was a quick and dirty project, so I just stitched up the side seams, turned it right side out, and then folded the fabric over the top flap and the top edge. I sewed those edges down with a decorative stitch. Then I added two sets of velcro to keep it closed.
The result, while perhaps way too flashy, is a lot of fun, and much more durable than the paper/laminate/duct tape version.

This envelope is so slim (that polyester crepe was positively filmy before I applied the interfacing) that it’s hardly noticeable in my purse — much sleeker than a fat wallet. (I use a small card case for ID and debit card.)
You, of course, don’t need an actual Dave Ramsey envelope to whip this up, but can whatever paper currency is in your wallet to gauge the size of your envelope correctly.