That's cheap. You seldom find drops in that size. They get turned into other things pretty fast. That price at McMaster is pretty cheap.
Do you need optically perfect pieces?
You could also get 2" cubes for $8 something and even larger ones, I think. I don't know what tooling you've got that you could use to cut that down to size. I wouldn't attack that with a standard saw. I've got a dedicated acrylic blade and a bunch of other specialty acrylic tools.
You can try any of your local acrylics places for drops, but places that deal in anything that thick are few and far between. I have a hard time finding 1" here when I need it.