You're not going to find Socorro anywhere else. That was B&S's name for what was essentially Van Gelder Simili Japon. I understand that this paper is either no longer made or no longer available in the US.

Another economical choice is Fabriano Artistico Extra White 140 lb Hot Press. Try Jerry's for pricing. This paper needs to be pre-treated with a soak in 2% oxalic acid for 5 to 10 minutes.