they are made out of polystyrene or high impact polystyrene (HIPS)
that's what i was told by this company Over The Edge- Wholesale Aquarium Tank Frames, Strip Lights, and Iron Stands
if you can't find a local fish store that can/will order you a replacement frame there's another place online that sells replacement frames Over The Edge- Wholesale Aquarium Tank Frames, Strip Lights, and Iron Stands