Re: Other materials for bottom of cage?
I would start by calling a few stores, cheaper and faster than visiting stores.
Call a couple of craft stores, ask them if they have plastic sign board. I've found some look at you with a blank stare when you ask for coroplast, but they know what plastic sign board is.
You might even try home depot, lowes or menards. Also look in your local yellow pages under the section for signs. Call a Kinkos/FedEx store. Some of these make signs and may have product to sell you.
I know this doesn't really answer the question of other items to use... but I thought I would post it just in case it opens a new place to look.