You will need to purchase replacement feet from an online vendor. Make sure the feet are for your exact model laptop. The feet for a MBPro are completely different than those for the Airs. The older Airs use different feet than the newer models.
All of the feet you can get are not that great (even the original feet are not that great). Most are plastic and not rubber. Some of the replacement feet will not be an exact match. Apple has small holes in the Bottom Case to use to line up the feet which have small studs on them. Many times the replacement feet won't have the proper alignment of the lugs. I just trim the tiny studs off the replacement feet so they fit better.
If there is a small piece of plastic foot left moving around inside, then it should be removed or it may become stuck in the fan. If there is a piece of the old foot stuck in the opening of the bottom case foot area, then it will need to be removed in order for the replacement foot to fit properly.
Don't expect a whole lot from the replacement feet as they are usually of very poor quality and poor fit.