For those who have a significant paper-based zettelkasten practice, have you considered commissioning custom made cards? There are a variety of stationers who do custom work and one could also purchase directly from Chinese manufacturers to get costs down by buying in bulk.
Ryan Holliday is one of the few I’ve seen in the wild who has mentioned custom making cards, usually done on a per-project (book) basis where he’ll put a header title at the top of his note cards. Example: https://www.instagram.com/p/CeWV6xBuZUN/?hl=en
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Other options could include doing custom/personalized stamps. (I have a date stamp handy for quickly stamping the dates of creation/updating in the corner of cards.)
I’m curious what suppliers/manufacturers folks have researched/used? What were your experiences? What sort of templates or printing did you use on them? Paper weight? Did you go Grid, blank, dot, lined, or all of the above? If you were looking to purchase something for yourself, what would you want?