any local glass shop should sell pieces of plexi cut to rough size, then you can sand to fit. If they don't have black just spray paint the side that will be in the case, it'll look a lot better than spaying the outside. Plexi comes with paper stuck to both sides DO NOT take the paper off until you are actually installing it for the final time.
I buy plexiglas at local hobby shops. Sometimes hardware stores also have plexiglas. If you have a variable speed jig saw you can buy a special blade for cutting plexiglas. I bought one at Ace Hardware for $3.00 or $4.00. Cut slowly and use low speed or else you will wind up with "melted" plexiglas.