You can buy a package of o-rings at a hardware store. The right size is the one that goes easily, but snugly, over the extractor spring.
They seem to be very durable. Obviously, if you do lots of shooting, the heat is going to break down the rubber sooner than later. Simply replace the rings on a regular schedule, like whenever you give the bolt a good cleaning. Only takes a minute and the rings are not expensive.
Join and support the NRA