Mine are Army surplus, purchased online. I have a second pair, practically identical, that I got from eBay. You can find them all over by searching for WWII leggings (they were not called gaiters). I had a thread on it a while back, but I'm on my phone so I can't really search right now for the link.

There are reproductions out there too, for those with larger legs who can't find originals to fit.