The only place that still stocks the cement around me is Ace Hardware, but most big box hardware stores should be able to order it at least.
The sheet stock I had to order online. A 1'x4' sheet of the stuff cost less than $15 if I remember right, and I still have most of it left. Just be sure it's actual ABS, and not stuff like styrene, PVC, polyethylene, etc. I just checked, and Amazon has plenty of it.