This might be the least expensive option:
https://www.amazon.com/DiversiTech-..._3?s=industrial&ie=UTF8&qid=1551855538&sr=1-3
But look out for the Supported weight rating: 50psi... if you put a sub with cone feet on these, you kill'em. Gotta put discs, or remove the feet.

I found a deal on the SVS soundpath feet. Expensive-ish though at $50 for 4 feet. But they flippin' work! Sorbothane pads might do the same as long as you double check the weight.