limulus: http://www.ezpads.com/ has info, google for dealers, such as http://www.poolandspa.com/page5114.htm
I'm looking at this thread as I will soon be moving my 6'x8' spa. I now have it on a pea gravel box but it was not exactly level after the gravel settled, and I want the new install better leveled, so no more pea gravel.
I'm debating between laying down a slab vs using the EZ pads, I'm inclined towards using the pads due to their relative simple installation.