If one has a lot of time and money there is Roy Fishpaw that makes custom handmade grips out of a variety of materials. He does not have a website. Hope this helps. All in all there are a number of alternatives.
R and R
P.S. At one time Ruger offered a wood grip for the GP-100 that matched the configuration of the stock rubber one but was all rosewood. Perhaps one could buy one on one of the boards.. Here is a link to a picture over at the Ruger Forum: