I wouldn't call it a proper "source", but the last 7-pin socket saver
I got was from Jim Cross @ http://vacuumtubesinc.com

I had to send him a note specifically for it, and he scrounged a
reasonable one up out of hits back room.

They've currently got a couple listed, but they aren't cheap.

His email is jim@vacuumtubesinc.com

(No affiliation, satisfied customer, yada-yada-yada)

David