I use both of these products for my amps, cd players, and turntables inside my bookcases, as well as underneath my speakers:
Neoprene and cork pads
https://www.globalindustrial.com/p/fasteners/vibration-isolators/pads/neoprene-cork-pad-4-x-4-x-1
Natural Rubber pads
https://www.globalindustrial.com/p/fasteners/vibration-isolators/pads/easy-cut-waffle-pad-natural-ru...
Rich
Neoprene and cork pads
https://www.globalindustrial.com/p/fasteners/vibration-isolators/pads/neoprene-cork-pad-4-x-4-x-1
Natural Rubber pads
https://www.globalindustrial.com/p/fasteners/vibration-isolators/pads/easy-cut-waffle-pad-natural-ru...
Rich