Are you looking for a grip that fills your hand better or one that allows you to have your pinkie on the stock? Or both? I have a set of boot grips that Altamont makes in Super Rosewood on my 637 that fills my large hand nicely. They also make affordable wood stocks in other styles.
As for rubber, when I was learning how to shoot the 637 I put a set of Hogue Monogrips on it that worked great! I did have to give up pocket carry though.