Alternatives to sewing machine bobbin felt pads
Bobbin pad? Or spool pad? I use spool pads to protect the surface of my antique sewing machine (1940's Singer Featherweight). Different shaped spools can sometimes rub on the surface around the spool area. I didn't use pads when I had a modern machine. 

Sometimes your thread might come on a different sized spool or a different size of hole in the middle, and a felt pad can help a spool with a larger hole sit better on the machine, and keep a constant tension instead of jumping around a lot. A friend's machine is very touchy about how it is threaded, and for shorter height spools a pad helps align the thread more with what the machine likes.

I bought my spool pads on Etsy and they are 100% wool and 5mm thick in a variety of colors. But you could likely use any felt.