X17-Mind-Papers - die Wiederentdeckung der Karteikarte<br /> https://www.youtube.com/watch?v=wxZMia35usc

Mind Papers has a variety of small leather covers (folders) with binder clips for storing one's note cards. They range from smaller than A7 up to A5 sizes.

They're broadly reminiscent of smaller versions of the Everbook, though I suspect these came first given the 2014 post date.

https://www.levenger.com/collections/pocket-briefcases/products/bomber-jacket-int-pkt-briefcase-w-pocketini-clip?variant=42636956237973

Try Kaitiaki or Rite in the Rain. If you search for "index card wallet" you'll likely find a variety of others, including some custom made versions on sites like Etsy. 3 x 5" are relatively common, but 4 x 6" are much harder to come by.

https://www.riteintherain.com/index-card-wallet