[C&K]

Linolium is a common substitute. You can get a remnant piece, or use sticky tiles on a piece of plywood. I would thing the remant would be easier to manage.

Chances are, you can find a coroplast source if you look hard enough. Most Home Depots carry it in the same section as sheets of plexiglass and such stuff.

There are some other PA members here, they may have a few ideas.

Another temporary alternative you can use is cardboard with a shower curtain or tarp over it. May not be as pretty, but it can buy you some time in hunting down the coroplast!

Let sign shops know you want it for a guinea pig cage, they are more likely to sell it to you if they don't think you are competition!