Hmm
I was lucky and found a commercial glass door from a building that was being demolished. it was 1/2" plate glass, but was only 32"x 80"
Maybe you can locate a window or something, and have it cut to size.
Cell cast acrylic, or better yet, cleat polycarbonate should be available from your local plastic supplier.
there are several around here, I imagine you should have some local to you as well. they can also help you with options for installing it, or maybe even make you a frame that you can install into the tank, and then install the poly into it.