I got the front and rear Williams Fire Sights from Brownells ($60 USD+). Consider me a very satisfied customer.
It is the same sight for the Redhawk and Super Redhawk, as well as the Super Blackhawk Hunter with the insert front sight rather than raised ramp, if you find any of those. Most GP100s use a very easy to change front insert, just push in the plunger on the front above the muzzle and it pops out and back in. Easiest sight swap you'll ever do.