I have bought some at sav-on crafts, e-bay, and pottery barn. I have made my own stands too. I have silver square ones at 14 and 20 inches. I have silver round ones at 12, 14 and 19 inches.
I also have several wooden, ceramic, and glass pedestal stands. If you come to the ICES convention in Charlotte this August, I am slated to do a demo on making your own cake stands!