Wot I've always known them as. A rubber 'foot' that screws on. Originally for between a wood toilet seat and the pan. Most hardware stores used to sell them in a variety of sizes. Usually 4 on a card.
In this case its a foot for an old Quad amp. Held on by a pan head M4 screw. Would obviously replace all four, as I doubt I'd find an exact original.
Search on seat buffers on Ebay brings up modern plastic toilet ones - usually rectangular. And glued on. Or spares for Seat cars.
Thinking I'd get them easily, went to Wickes. Nothing. Looked online at B&Q and Screwfix. Nothing.