Pro hint: most of what's out there is either for 3x5" or 4x6" cards, but if you're folding them to 3x4" then check out not only the 3x5 solutions, but look at what the same manufacturers of 4x6" solutions offer and see if you can scale down. For example, Lochby also has a folio for the standard 3.5x5.5" pocket notebook which makes for a nice and smaller sized wallet than their A6 folio.

You'll find a lot more custom made stuff in the 3x5" range, but can find other carriers, wallets, etc. made for other things that you can re-purpose for your 4x6" needs. As an example, I've found some interesting things from companies like Rickshaw made for carrying fountain pens, but which are comfortable enough for 4x6" cards.