If you're looking to replace the wood inserts in the factory grip yours is currently sporting, I don't know if such a thing exists (or is even possible) as mine is the slightly later model GP100 that came with the Hogue rubber grip (all rubber, no wood panels). As far as replacing the whole grip on the GP100, there's several places that sell many different kinds of grips for it. Only the style of the grip has changed from your model to the later one. The way it is attached, and the shape, is identical.
EDIT: Just came across a site in my searching talking about switching the panels in the grip. That's kinda cool, I didn't know they were separate from the rubber. I have no idea if those panels are readily available to do that or not. A google search of GP100 grips resulted in many places selling whole grips, but I didn't see any panels...