Try the following website:
http://www.mcmaster.com. It's the web site for McMaster-Carr, distributors of a variety of hardware and equipment, who will sell to individuals directly.
The item you want is Part Number 9376K112. It is a Vibration Damping Mount, with metric threads M4x0.7, 1/2" tall by 5/8" in diameter, at $1.66 each. They are just a little larger than the originals, but they work just fine. There are also the same item in a slightly smaller size, 3/8" x 3/8", with the same thread, if you prefer. I didn't write down the part number, but you will see it on the web catalog page just above the part number I provided.
McMaster also carries well-nuts, which I also bought from them, but unfortunately they don't carry metric threaded ones. I just replaced all of the standard metric well-nuts with English threaded ones, and replaced all of the mating screws to match.