They are called "Bar Service Mats" and you can get them in varying sizes and colors and even with your favourite adult-beverage logo on them...eBay has them.  They can be cut with a sharp sheet-rock knife and a straight edge to get the size you want.
 

Alternatively, my standard solution to replacing typewriter feet is to use readily available, off-the-shelf, and inexpensive rubber bumpers found in hardware stores. If you order them online, there's a broad range of shapes and sizes available.

Call me cheap, but I personally can't justify spending $25 (assuming it's USD? = $31 CAD) on replacement feet for a machine that cost me $35 CAD. A set of bumpers only sets me back a few bucks. Clearly I'm not concerned whether or not the feet look original - function is my primary concern; the vast majority of the typewriters I own are not rare and unoriginal feet won't change their resale value.

Typical hardened, flattened, or otherwise deteriorated rubber replaced with off-the-shelf bumpers:

The last set of bumpers I bought came from Amazon, and cost $18 CAD for 48 pieces. When it comes to models with odd-shaped feet I usually find some way to make the standard round bumpers work. Again, this isn't difficult when you consider that I'm not concerned with how the feet look, or maintaining any semblance of originality. Even more complex feet that use a compression fit can be replaced with ordinary bumpers and some adaptation work with a razor blade. I only mentioned this because I assumed there are other collectors who are as cheap as I am and don't like to invest money in an inexpensive typewriter.