Like mentioned above, guys like Hamilton Bowen, John Gallagher, Alan Harton and others all deck grip frames when the service is ordered a part of a larger project. If I just wanted to send a grip frame out for decking I would check with Andy Horvath at Diagonal Road Gunshop in LaGrange OH. He is dong a barrel and hammer swap and action job on an old model flat top for me right now and he seems to have some of the best turn around times.
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Place a piece of 220 grip sand paper on a perfectly flat surface, like a marble counter, pane of glass, table saw surface, etc.
Rub one grip across the flat paper holding the grip flat. Observe the sanding marks. If the marks aren't across the entire inner surface of the grip, it's not flat. Keep sanding 'til it is.
All of Ruger's Bisley grip frames are notorious for having a high spot on the grip frame. You can see this high spot in the pic below. It's marked with the X. Any time a customer sent me a Bisley grip frame for grips, I always ground down that high spot so the grips would sit flat. It's easy to do with just a dremel tool with a sanding drum or a grinding wheel on it. On the blued frames I'd just touch up the spot with some cold blue. Of course you can't see the spot when the grips are on the gun anyway.
In the pic, you see a piece of tool steel next to the high spot. If you slide that steel up, it will catch on that high spot. You can see that I've actually ground this one down already. That high spot is what gives after market grips problems sitting down on the frame.
The other pic is one of the Bisley frames that Hamilton Bowen sent me for grips. You can see the beautiful decking job he does on them.