I made some with a rubber stoppers from the hardware store... Cut it down to size and diameter, and epoxied them on. Could not tell any difference.
If you really wanted to dig into carving, you could probably even make them with the mounting plugs too.
But yeah, if you have a local junkyard with old stock you can get a ton of little parts like that for probably less than 5$. Last time I went to a yard I got extra rubber for each lid, door, anything that had it. Also pulled tons of clips, fasteners, vacuum lines, other rubber that could rot of break and fall out.