If the socket is still functional, I'd just RTV it to the housing. The plastic is old and just breaks down over time.
If it doesn't work, you have a few options.
1: Find a side marker assembly online and use the socket outta that, but again with the plastic being so old, it's a shot in the dark if it's lugs are broken or not.
2: Buy a universal #194 lightbulb socket (Amazon, eBay) and RTV that to the housing. You may get lucky with some searching (I found some Loyale sockets with a Google search along the lines of "Subaru GL 194 lightbulb socket".)
Good luck on your search!