There are lots of tri-five Chevy stuff at swapmeets and 4 door parts are usually cheaper than 2 door parts. I've seen plenty of side glass. Not many good windshields though. Maybe they don't bring them cause they don't want to risk breaking them so you have to ask.
Flat glass isn't very expensive. I had a new window cut for the drivers door on a 57 Chevy 4 dr at a local glass company and it cost $30. I had six pieces of dark tinted glass cut for my 40 panel for $165. It has a 2 piece flat windshield, 2 front door windows and 2 rear door windows.
If you have a cracked window to use as a pattern they can make a copy. If you don't have a useable window you can make patterns with a piece of cardboard.
While you're checking out 57 Chevys on ebay do a search for "1955 1956 1957 Chevrolet window glass"