The standard ones are usually between 12 x 12 and 12 x 16 - they're often rectangular rather than square - and I set them up on a pair of cinder blocks; those are easy to get at home improvement stores.
I set up my boards on cinder blocks from Lowe's. I'll check to see if they have those roofing slabs...
meanwhile, I'm also interested in any insight anyone has on protecting your little finger from bruising/slicing on a knifehand break. I'm pretty sure that the only contact I make on my knifehand is with the muscular ridge between the base of the little finger knuckle and the palm/wrist line, but in the course of the break I find that sometimes the top board (I think it's the top board) collides with my little finger—maybe bounces up?—and cuts it on the side right at the first knuckle, or else leaves a nontrivial bruise there. Does that happen to anyone else? What do you do about it?