Lowe's and Home Depot usually sell plexiglass. Unless you've got a good tablesaw, don't try to cut this crap yourself. It's a big pain in the butt. Have them do it for you. Then round off your edges with a dremel tool.
I think ZTrip made a thread about it, try searching.
Re: plexiglass, I'd stay clear of it for lizards. Their claws will scratch it up and it will look like crap soon...though maybe a beardie isn't big enough? To be safe I'd try to get some glass instead, have it cut to size for you and you can put tracks for it to slide on right on the frame.