back in the early 90's, ruger produced the sp-101 in 9mm............i'm not sure when it was discontinued......
the ruger moon clip ad is not a typo, it states it is for the 9mm revolvers and can't be used to convert a 357 to a 9mm.
moon clips for a 357 sp 101 can be obtained from several sources and are for a sp-101 that has already been custom modified by a gunsmith to accept the moons. Involves removing metal from the cylinder.
here are some sources............
these outfits will do the conversion
Moon Clip Conversion - Clark Custom Guns, Inc. Pinnacle-Guns.com: Pinnacle High Performance Custom Gunsmithing
and these will sell the moons
Ten-Ring Precision, Inc. Welcome to TK Custom.com & Moonclips.com
hope this helps........