I have exactly what you're looking for! I bought mine on eBay and they were something like $6 each. I have a red one with Mickey and a pink one with Princesses. It is a small (slightly larger than a credit card) coin purse with a clear plastic area on the back for your card. The lanyard is a matching color and print. There is a plastic clip to detach the purse from the lanyard. Or you can detach it using the metal lobster claw clasp.
Everytime I go to Disney I have several people ask where I purchased it.
I just looked on eBay and the pictures only show the front of the coinpurse, not the clear pocket on the back. Sellers are calling it a Fastpass pocket. Lanyard with wallet set is the name I saw.
I love these. Do you know if they sell these at the Parks? Laughingplace.com has them so that is why I was wondering. I'd love to save on shipping if I can just get them when I'm there.
Well, I can honestly say I've never looked at Laughingplace.com, but after some searching I did find them!
http://www.laughingplacestore.com/Category-RC-601.asp
They are $8.29 (plus shipping $3.45) and they have Red Mickey, Black Tink, Light Pink Princesses, Blue Mickey and Friends, Black Cars, and and Red Cars.