I am only just getting started with my Zettlekasten, but I thought it was worth mentioning that contrary to what you wrote in the beginning of the article, A6 is a perfectly viable size format even for American users, as (in landscape) it is slightly narrower than 4x6, and only 0.13in taller. So any box which would fit 4x6 alphabetical dividers should also work fine for A6. I just bought one of the fireproof Jundun ones.

Not sure how affordable it is to get “cards” in this size, but I am using A4 sheets of Kokuyo KB paper cut into A6. This produces 0.09mm thick slips that can still be written on with fountain pens, and each cost about $0.01. Once you try Japanese paper it’s very hard to go back to anything else.

Great article though, I suspect I will return to it in the future when I have a more permanent place to put things.