I think Home Depot has it. Lowes definitely has it. I worked at a Lowes for a few months once. The may
have some scraps big enough for a stompbox they'll give you for free. The store I worked at was cool about that. They're just going to throw it out anyway. You could have them rough cut it to close to the right size. The cutting machine always seemed to be a little off where I worked.
If you want to cut it yourself, score it with a sharp utility knife, and snap it off. If you try to cut it with a Dremel cutting wheel, it will melt the plexi, shatter the cutting wheel and send shrapnel and melted plexi bits directly into your eyeball. Wear eye protection.
Anyway, a decent place that sells plexi should have some scraps to give you to practice on.
2 questions about this process....
1. Where does one find plexiglass? Would the average super-hardware store (ie Home Depot) carry it?
2. How hard is it to cut? Does a Dremel work well for the task?