Try calling:

EDS Sport Shop
308 W Broad St
Tamaqua, PA 18252

(570) 668-1080

They do a lot of gun shows in Southeastern PA and usually have a couple of tables devoted solely to magazines, including many that are no longer manufactured. If they can't help, they may have a suggestion of where you can get it. I've bought a number of items from them over the years, including some stuff for long obsolete rifles, and have always been pleased with their service.