Late I know, but you may want to look through Etsy for some that might fit your requirements. It’s where I found an excellent waxed canvas notebook pouch with pockets on one side and slots for pens on the other. (see: https://i.etsystatic.com/11030599/c/2250/2250/523/0/il/3270c1/3501351051/il_300x300.3501351051_co7b.jpg – it’s no longer for sale but I see similar there).

Am new to the Zettelkasten thing but am looking forward to it assimilating me. ; )

Ray