I also got my 20 pin socket from OETECH where John Folsom got his. I ordered some old RCA phoatomultiplier tubes for the male end and they also had a Female socket. Cost was about $80
I did a search and found this company.
http://lancosockets.com/
It looks like Lanco has sockets for some 20 pin PMTs but I do not know if they are the same as the one on the 15GP22 There is a diagram on the Lanco site of the 20 pin socket they offer. You will have to figure out if it is the same as the 15G base.
Good Luck