They're called rubber grommets (to distinguish them from the hard plastic or metal kinds). You can get them at hardware stores, or online vendors like McMaster-Carr in the US. The important dimension to consider is the diameter of the hole you drill to mount it in.
You can even buy them with the appropriate military specification number (see MS35489J) if you feel like paying a whole lot extra. You will get a nice set of material certifications that you can frame and hang on the wall.