You're going to want plastic, that's for sure. Stone will get chipped and degrade, while wooden ones don't weather quite as well. Personally, I'd suggest getting a bunch of sets from HOS, which are quite cheap. Exactly which ones to get will depend on the size of the squares on the board.
EDIT: Assuming you have standard 2" squares, I'd recommend these as a cheap option. There are also some triple weighted sets for a few dollars more.
Hi everyone. At the school I work at there are outdoor chess tables, but no pieces. I was wondering if someone could point me in the direction of where to find some options of pieces for outdoor play. I know I could just throw any plastic pieces out there, but I was wonder if there are any popular chpices used by public chess parks.