I have a tiny "Toyo" lathe + mill, and the number of collet available is limited to 6, 8, 10 mm, (not to mention the price !) and only compatible with themselves, neither ER or OZ...
I had to make my own for holding 12 mm mill bits, which I have in a large number for my Larger X2mill.
I was not that easy as the final collet is extremely thin, as the largest collet is 10 mm, but once installed, it holds perfectly the 12mm tooling.