I have old cross country skis which require those boots with a flat end with three holes which you insert into three pegs on the skis. I'm looking for the name of that style of boot in order to be able to find some on craigslist or similar websites.
I think those were the original 3-pin Nordic Norm bindings.
75 mm (Rottefella, Nordic Norm, 3-pin) This is the original, classic system found on cross country skis, invented by Bror With. These bindings, once the standard, are no longer as popular as they were but still hold a significant share of the market for mid-weight touring setups with relatively heavy boots, as typically used for hut-to-hut touring in Norway. In this system the binding has three small pins that stick up. The toe of the boot has three holes that line up with the pins. The boot is then clamped down by a bail. Despite the decreasing use of the 3 pin "rat trap" ("rottefella" in Norwegian) binding in lighter cross country, the characteristic "duckbill" toe it uses is still assumed in the design of heavier cable bindings, and 75 mm boots are still widely available, especially for telemark technique and more rugged touring.