Guns R US, the local store, has one, for $9.95 + the govt cut. Brownell's will be cheaper, maybe faster.
One thing, when you put it back together, clean it meticulously, and use a moly grease wiped on,wiped off, a al Karate Kid. Less stuff for grunge to cling to.
The smith down at GRU said that almost nothing goes wrong with a properly maintained 870, the big killers are abuse and neglect. He did say repeated dry firing wasn't a great idea. Another spokesman for snap caps...