Hi guys,
The short answer is no, there are no commercially produced retarders in O; at least not functional retarders. There are several people who have modeled hump yards in O, both 3-rail and 2-rail, including forumite Bill DiMenna. I'm not sure what, if anything, Bill used for retarders. Most folks who have modeled a hump yard in O have done it without retarders because scale cars do not coast like the real thing. I have seen successful retarder systems in HO using both compressed air (nozzle blowing against the cars as they pass) and magnets (artificially "weight" a car with steel plate at is passes). All the systems I have seen in person were scratch built.