We offer leather vegetable-tanned leather cards, which are based on historical finds from Norway and Ireland. They are 1.75" square, with rounded corners.

The cards are 2-3 oz leather (1/32" thick) and are cut with a die, so they are all the same size. They are not coated or hardened - they have the same flexibility as playing cards when warped, and don't require anything extra.

These are what I personally use for 99% of my tablet weaving, and I will provide the documentation if requested.