Amazon sells machine plates at a reasonable cost. If you are lucky and the dimensions fit your speakers footprint, you might be able to score a pair for an acceptable price. Otherwise, going to a local marble dealer would be the next alternative.
I own Vandersteen Treo's. Mr. V. swears by adding a 3 inch granite base allows the speaker to perform optimally. I own the Townshend speaker podiums, but added them to my components instead of the speakers, where they have clarified the bass significantly.
Bob