P&S Militaria is another good spot to keep an eye on.
And of course, there's Collector's Source......but be prepared to part with an arm an quite possibly a leg. CS pricing has been completely out to lunch for a fair number of years now.....
Marstar keeps a used firearms section as well. Fairly prices Lee Enfields tend to get snapped up quickly, be ready to buy if you see a nice one....
Cabela's gun library seems to pick up one or two on occasion as well, at least the local one here does.....
Last (and probably best) bet, hit the gun shows. There's always a few old military rifles kicking around.