there is a guy here who runs a junk / antique shop and he sells granite slabs from custom counters (usually the sink cut out part) for $20 / per.
they weight around 60lbs, and are beautiful. I have a matching pair that I used as amp stands, but have since put my speakers on them (big difference, btw...better than spikes).
I was thinking about getting 3 more of them and trying to make a rack, but am scared about drilling into it w/o them shattering or getting ruined somehow.