either Aluminium LM6 strontium modified rim Alloy or Aluminium Alloy 6082. There might be other metals out there. I'm with you, I'd stay away from plastic.
I got a question though, what made you decide to buy Rondell? I'm in the market for wheels and was thinking about buying Rondell #21's. Donno much about this brand other than some members here have used them before.
Check this site provided by Jason to see if they have what you're looking for:
http://www.thewheelexchange.com/
Hector