Many gun stores now carry some of the holsters used for cowboy action shooting stuff. I looked at some there. I have seen some advertised in different catalogs like sportsman's guide as well. And many of the holster makers listed on tfl offer the cowboys tuff.
Finding a leather belt holster for a redhawk was beyond one gun store, they could not find one for it. Blackhawk, and my superblackhawk were easy as single actions on a belt seem more normal.
Run a search for cowboy action shooting both on this site and the net. Beartooth bullets has a section on their site and there is another site along the lines of sixgun or something I think. A search of this site should turn up working links.
A search for holsters should come first.
It is not terribly hard to make a holster for the gp100, and the belt is holding cartridges and the holsters would slip on and off so you install what you want.
Best depends on several things. The more expensive holsters have patterns and stuff on them, all I want in a belt is nice thick leather in a solid design and well worked so it has no further problems. Same with a holster.
I think some window shopping will lead you where you want to go. The cowboy sites most likely could offer far more advice than I have here though.
If after the truly best and you know what you want, a saddle maker in your area could make a belt, but you will pay for it of course.