Call and talk to Rick Sanborn at Green Mountain Barrel Company. I am sure he knows who makes one that would fit. As the other poster said, I think they also have them there. Seeing that they are technically the same people that make Knight Barrels, I would think the same sights that Knight uses would have one that would fit. Knight used to use Tru Glow. I am not sure what they use now.
Look at Tru Glow Fiber Optics. They are a great fiber optic. If you e-mail them with the barrel information in question, they will tell you which one of their sights fit. I am still using the sights that came with my barrel.