Hi Chris: Check these sites:
I believe Home Depot and/or Lowes sells the steel for this.
Your local hardware store should be able to help you.
If there are some air conditioning contractors in your area, my client used to recycle them and there were some big panels and some medium ones when he would disassemble them. Talk to a local contractor and see if you can buy some cheap. You might have to strip some paint off if both sides need to be clear.
We bought some at Home Depot so I'm sure most big box and hardware stores carry it.
I would check a local metal worker shop to see if they have it in rolls and can cut it to the size you need.