Vibration isolation pads that are used under mechanical equipment (i.e., pumps, fans, etc.). My father had a commercial/industrial HVAC business for 35+ plus years and they kept a small inventory of them in different sizes and load ratings in the supply room at his fab shop. I would do a little "shopping" in the supply room every now and then. I have them in a bunch of different sizes and thickness.
I can't remember the name of the supply house that they got them from, but Grainger has them....just search "vibration isolation pads"