Sarge (AKA the Finer Grain) can set you up with Stag (or Ivory) grips.
Nutmeg Sports is also a premier place for Ivory (although not sure if they handle Stag).
Dan Chinnock Knives (but he also sells custom grips) is also a premier dealer. He can set you up with 100% Sambar Stag, Elephant Ivory and 100% genuine Mammoth Ivory from Siberia.
All three are top flight/premier dealers and all can be bought over the net.