You can get verious grades of plexi-glass and acrylic sheets at Menards. There might be cheaper around I never searched to hard fot it. They also sell a tool that is a buck or two. It is very simple to cut. Use a stright edge, score the acrylic, and snap it.
There is a bit more to it, but that is the jest of it. You will want to score the acrilic several lines, but make sure you stay on the same line. Only use slight pressure, but several strokes across the sheet. Keep your stright edge on the line whe you snap it off. Holding the strigh edge on the sheet have the scored makr just pass the working surface and snap it. Also make sure the edges are well scored. That is where I usually have a problem. If there are places it does not snap off clean, use a pair of pliers to gently snap it off, small areas sand off easy.
Glass is easy to work with and adhears well as ballfles, but you will want to go to a shop that can round the corners so you don't slice yourself open doing mantance.