Try the Ruger parts department.
Grip Panels - Sold in Pairs Only - Part# XR01000
Sturm, Ruger & Co., Inc.
Customer Service Department
411 Sunapee Street
Newport, NH 03773
(Monday through Friday 8:00am - 5:00pm EST)
Telephone: 603-865-2442
Otherwise, you will be looking for a grip commonly known as the XR3-RED which fits the old original Vaquero, the current new model Blackhawk, the current super Blackhawk with 5.5" and shorter barrels and single-six WITHOUT INTERNAL LOCK.
rc