Also McMaster Carr in robbinsville, nj carries it.
That stuff is expensive as all get out. I think I remember seeing sheets in Home Depot you could get as 2 x4 or 4x8 up to maybe 5/16 thick that was last year that I saw them in Absecon so I am pretty sure the other stores carry it a well.
I didn't see it in Home Depot by mi I have to look around tomorrow when I got there.
Not sure where you're located but there is a place in Philly called everything plastic right by the WW bridge. They should have what you need