I’ve never seen them for sale anywhere but we used to mount tiny 14” Goodyear Wranglers on postal vans.
I looked into this recently and from what i remember the answer is they are hard to find and expensive when you do.
Another problem is that even tires that seem small are too big for a fit. Most of the real all terrain type stuff you can find starts at ~27" diameter and goes up. There's VERY LITTLE in the 24-25" range that isn't an actual rally tire. I think running snow tires and just avoiding overheating them on pavement is the best compromise answer.
