Granite surface plate

I did exactly what you propose. 3" granite surface plate on a machine stand from a company called Production Tool. Works fantastic with my VPI HW-19. Grizzly sells them nationwide. You don't have much to lose.
Try this and the next page.
http://www.grizzly.com/search/search?q=granite%20surface%20plate&rankBy=relevancy:descending&page=1

If you are serious about a turntable platform you will acquire an electron microscope base. Most go for about $10K. Mine works well.

Seriously mentally ill that is.

I was planning to place the surface plate stand on a carpeted concrete floor. The stand is a machinists' stand specifically designed for a granite surface plate. It has adjustable leveling feet.

I had also read that a thick maple cutting board on a granite surface plate works very well. But I can't remember where I read it or if the credibility of the commenter.