What are these things called?

[WinDancer]

There are various holes in everything made. Sometime what goes in the hole doesn't come with it, so the use a little round rubber or hard plastic piece to close the hole and make the product look more finished.
I see quite often here on the board ads for 'delete plates' that seem to serve the same purpose. Also see 'block-off plates' for fuel pump conversions, electric water pump conversions and may, many others.

I am after a source that carries the round rubber ones [in a bunch of different sizes] with the lip cut into the outer edge so it fits tight on both sides- you press it half way in and then work the rest of it around the edges until it 'snaps' into place and seals the hole.
Any leads appreciated!