All Shimano internal-gear hubs use standard 3-spline sprockets, same as most other internal gear and coaster brake hubs.
In addition to the genuine Shimano Nexus (works with Alfine too) sprockets, that will work with either 1/8" or 3/32" chains, we also can supply most sizes in the Sturmey-Archer version, which is for 1/8" (wide) chains only.
Note that the Nexus and Alfine hubs require a dished sprocket, so sizes smaller than 16 teeth are not suitable for this application.
Nexus Sprocket 16T
Nexus Sprocket 18T
Nexus Sprocket 19T
Nexus Sprocket 20T
Nexus Sprocket 21T
Nexus Sprocket 22T
Nexus Sprocket 23T
Sheldon Brown's Internal Gear Calculator for Internal Gear Hubs