I have a 1989 Conn 6D, which has much smaller rubber stoppers (bumpers) than the old 1950's models. The horn plays beautifully and hasn't the problems of the 1970's/early 80's, thank goodness. It was nearly pristine in fact, given its age. I've tried 2 kinds of replacement bumpers through a dealer on ebay, but neither worked out. One was much too large circumference-wise, though it was for Conn. The other type was a fool's hope. The seller and I discussed that it was only made for Holton and the like, and not for Conn--we gave it a chance and they were just a hair's breadth too small.
No squeezing or stretching with pliers or a vice would work to make either choice eventually fit the inside of the stopper plate's notches. Meanwhile, the two that are desperately in need of replacement are increasingly causing clacking noises during play. Yikes.
Does anybody know of an actual corporate/professional supplier online? The ones I have have "arms" but I imagine I could also just use cylinder-shaped stoppers.
Try writing to Osmun music, they carry lengths of BUNA-N gasket material (I'm guessing you buy it through a machine shop supplier as well or some such)which is perfect for most horns. When I had a tough time just like you, I called them and they had stuff shipped to me the following day. Got here in about 3 days. http://store.osmun.com/browse.cfm/buna-rubber-rotor-stops/4,1528.html
I ordered from them under their supplies section. The thing came so fast and in a bulk cord. All I need is some time this weekend to dissemble the plates and mechanisms. We'll see. Like the guy on ebay, they had the rubber stoppers listed as generally good for Conn. However, Conn has had such an uneven history and ownership, especially in the 70's and 80's, that even a very good sounding intermediate horn like ours is not the same size in the corks as the classic 1959's, etc.
I hate those arms, and the horn I play needs a softer action, which Osmun offered. Just the best service. The material they offer is hell on blades, don't know what that's about, but I went through two single edge razor blades for the 4 valves just to keep the cuts smooth.