I believe you want Picatinny rails, as that cut will also take Weaver-style accessories.
Brownell's has various lengths, and I have seen individual lengths being sold in various ads in Shotgun News.

All things considered, I would think that metal rails would be MUCH preferable to some other sort of material.  Metal mounts can be much more securely attached to the firearm, and the mount-to-sighting-device interface will hold up longer as well.  IMHO, plastic rails are almost always a false economy.