If you want to make one, these are super easy to measure, cut and sew. This wallet will carry (comfortably) 8 cards, plus 5 bills. This is made from black textured vinyl which is thin enough that I could run through the sewing machine.
Measure 4 6/8'' x 3 6/8''
At the 2 3/8'' cut a hole to your liking. Mine was about the size of a nickel.
Measure 1'' x 3 6/8'' This will be your strap. I measured 1/8'' and beveled the strip as a decorate measure.
Cut your wallet design in the fabric of your choice. I would recommend using a wallet that has some stretch as your cards will need to fit inside it. Fold your fabric right sides together and slip your strap inbetween the two layers at the midpoint. Carefully sew along those edges at a 1/8'' seam allowance on both sides. Invert everything so only the right sides are showing. Use a pen to even out the corners. Wallet is done!
I measured the midpoint on the wrong side of the fabric and added a decorative stitch.