The two popular ones for the RC51 that I'm aware of is Scotts and GPR. The GPR requires their GPR fairing stay, which adds to the cost. The scotts just fits with the stock fairing stay.
I only have personal experience with Scotts, but I think it's great, and I give high marks for Scott's personel. I talked to them on the phone a couple times, (which is saying something in an age when 1/2 the companies out there have no-one to answer a phone), and was really impressed with the impeccable records they keep on their products. Even though I bought my damper used, they were so friendly and helpful.