It's possible to make leather - even wallet leather - much thinner by "skiving." The best way I've found is to soak a piece of leather in soapy water, let it dry until it's just damp, turn it rough side up - smooth side down, and slice off thin pieces using a very sharp, curved X-acto blade. It's like peeling a potato with a sharp knife, except don't try for long strips. Keep skiving (slicing off thin pieces) until the face is as thin as cardstock.
In case you're tempted, I've tried using a potato peeler, and it doesn't work.
Leather that's still damp with soapy water will slice like butter, so it's surprisingly easy to control your cuts. Don't expect the result to be uniformly thin, but you can get it pretty even.
It made me nervous the first time I tried it, because I was worried I'd cut all the way through and ruin the piece. But won't cut through as easily as you might think. My guess is that tanning toughens the face more than the rest of it. Be sure you have more than enough for the job, just in case.
Very thin leather is stretchy stuff, so cutting and fitting it can be tricky. So it's also a good idea to skive some extra pieces.
Ddms