Hi,
I'm constructing a floating floor for my humble homestudio and I'm using these ones (see attach). It's used to put under big machines (industrial application), it carries 30kg, has a hardness of 57° Shore (between hard and soft rubber) and costs about 4€ in Belgium.
It's from Overtoom International, a supplier of office furniture, industrial stuff etc... but they cover mainly Europe, so you'll need to seek for a US equivalent supplier.
Just thought I give the info.
Greetz and good luck,
The Niz