Thanks Roger. I was able to find an online company called amequipment.com. It doesn't look like they sell just the part I need, I'd have to buy the whole arm, but if that's the case I'll do it.
Which brings me to my next quandary, how do I remove the entire arm? On a car there's usually a 13mm nut under some sort of cover, but it doesn't look like mine works that way. Here's a picture of that. I don't see any obvious way to remove it. I guess I can ask the company when I write them.
Mike: just saw your post, yes that's what I'm seeing.