We’re always on the lookout for interesting paper stock for our printers; most of the time, though, it’s paper that actually goes through the printer. Last fall, while wandering through our local photo store (Pro Photo Supply, if you’re in the Portland, Oregon area), we found some postcard stock from a company in Wisconsin called Romar.
Romar’s Post-Pix are definitely from the pre-digital era. They have a paper protector that hides a strong adhesive on the front of the card; you simply remove that and affix any 4" by 6" photo in its place. The result is a nice, thick postcard that holds up well in the mail, and makes a great statement with your own pictures.