Home Depot carries plexy don't recall the size but large sheets up to 3/8" probably. Otherwise maybe easier to find online. What Size you need? I might have some scape thin stuff in the barn.
Windshield? My experience would advise against this, for most cases. JMO
good luck with the plexiglass. i had some on a rail many years ago. i spent more time looking over or around than through. the plastic scratches easily, yellows and cracks. then there is the problem of having to have a windshield wiper. (law) again good luck.