Gas station light fixtures, porcelain reflectors and complete pole lights are increasingly found at petroliana swap meets. Often the parts are in boxes under a vendor's table. You have to root around. I have purchased several reflectors and fixtures at recent petro swaps for my project. I got some reflectors for $10 to $25. Complete fixtures go for a lot more.
A list of petroliana swaps can be found on this site's Events page. http://www.oldgas.com/pp_swaps.htm