I got mine from a local commercial glass place. I got 3/16 thick. Just remember to measure 3x then cut once. I left the protective film on and used a jigsaw with a metal blade to cut it with. Mine is on a DDable 4runner.
There are other options, (Speedglass comes to mind) that won't scratch so easily. I had Lexan in my Truck that was DD for a while as well as off road only for a while. It got so foggy that I couldn't see through it. Others on here will chime in with other products that I can't remember.
Lexan is awesome and strong as can be but it does scratch very easily, like I mean if you took a paper towel and rubbed it across it would leave slight scratches. If its a strict trail rig I would put a regular windshild in it and then pull the windshield off when I wheeled. Or just put the lexan in there and clean it very easily with the softest cloth possible. I have worked with this stuff alot and I would use 1/4'' thick.