I had to replace mine on a '91 HR. I did a google search for that type of arm and found a place in WA St., the factory. Can't remember who but I think there were just two choices.
Got their service guy. Wasn't expecting much. He needed the exact measurement of each component and a little other stuff. Turns out they probably originally made it or knew who did. He said they would have to make it and how fast did I need it. No hurry I said. He asked if a week would be fine, I said wonderful. He asked me to go out and measure again before we completed the order. I order a pair.
In less than a week I had them including all the nuts and bolts. Turns out they make those systems for the RV industry, fire trucks etc. and he even was aware of the model I had. It was the most pleasant and expert buying experience I ever had and I told him so.