Try Springfield Sporters, Empire Arms, Southern Ohio Guns (SOG), Century Arms, and one or two of the Lee-Enfield discussion forums, it seems there's always a No4Mk1, No4Mk1*, No4Mk1/2, or No4Mk2 for sale by one of the members.
Although, for being as cheap as they used to be, it looks as if the supply isn't getting any better, and prices have been climbing on the Lee-Enfields slowly but surely.
I'm up to 4 of the darned things now, and am looking at a fifth, they kinda grow on ya after a bit...
Let me know if you need any help locating these dealers/sources.