Tom's source may prove better, but sign supply houses carry that stuff, but I forget the dimensions. In this area, PA, NJ, MD, NY..., try Harbor Supply.
I don't think acrylic is sold that thick around here. I've bought 1/2" if I remember correctly but for 3/4" sheets, polyethylene compounds (HDPE, UHMW) were my choice. They don't typically come in various colors either.
Suppliers to sign shops are definitely the place to look. You could start with a sign shop and ask where they would look.
We have several Canadian memebrs who might benefit from knowing you, Now that you've popped up, please start a new topic so you can tell us about yourself and your company. And please include some picture - we LOVE pictures!