Most people will tell you to cut your own acrylic (google acrylic sheets and you'll see a lot of places that sell black sheets). I, however, am lazy and terrible with tools, so I just got Lifereef to cut me one.
Sign makers should have some also. Just look them up in your area. You can use a dremel to shape them up. But if you want a "nice" one, go with Len's idea.
