I found rubber grips for my Service Six serial # 159-XXXXX, made in 1983, at Hogue. My gun has the flat bottom grips not rounded. I don't recall the model number exactly ... but probably #87000. I paid about $25 at a LGS.
Good luck. Enjoy the gun. I love mine.
I bought a set a pachmyers and used a dremel to fit them. I still had a little separation so I then super glued them together and they work fine. They seem to hold up under recoil so far. If I decide to take them off I'll probably have to cut thru the glue.