Since I don't know the specific sizes or quantities I will give you a pretty general list of sources:
Steel: Triple S steel or Rose Metals. There are a bunch more places around Houston but these two will cover all your needs. Also they sell surplus steel at a discount.
Plywood: Can't beat Home Depot or Lowes for run of the mill plywood. If you want fancy stuff like Baltic Birch plywood try Houston Hardwoods.
Plastic: This covers alot of ground. You can always mail order through McMaster Carr. Great selection and they ship quick. Same goes for rubber also. There are probably locals sources for plastic and rubber but I have not dealt with any of these locally.