Circular moulded rubber feet from Affix for use on instrument cases etc. A metal washer is incorporated in the moulding which prevents the fixing screw head from pulling through the foot.
This are exactly what I need to put under my 3d printer. They have a metal washer inside that will suit an m3 screw. The really good things for me was that the rubber is not too soft but soft enough to dampen vibration without causing wobbling.