The Alex Henry and Rigby grip caps are the same as Brownells sells. The one that came with Don Brown's kit was the Paul Dressel one. I also use the one by Leonard Brownell and keep one as a template to trace around when making them from other materials.
This is the Dressel one.
This is using the same one as a pattern for a skeleton cap.
This is an ebony one on a Rigby.
I also have a current build using water buffalo horn, but don't have pics of it yet.
Other than steel, the grip cap is usually from the same material as the nose cap. They aren't hard to make and anyone with a modicum of talent should be able to do so. Everything doesn't have to be purchase from a supplier