These are used w/ carpet spikes, they look like little pucks/disks.

I can't seem to google them correctly...

I have a set of 8 that I'm not going to use. If you want them you are more than welcome to them.

Take Wid up on his offer, or if you want to buy your own...

I bought mine from here. Under the 1.375" spikes, you can order just the 'floor cups'.