I don't know how much of a DIY guy you are, so this information might be useless to you, but this is what I did. I ran into the same problem finding a ball mount for my accessory rail, so I just made my own. The base is just a piece of 1" wide aluminum bar stock from the hardware store. I also bought a U-bolt with sufficient internal clearance to fit around the accessory bar. I drilled a couple holes in the aluminum bar to secure the U-bolt around the bar and into the aluminum base, and this was topped off with a couple acorn nuts. The U-bolt has some shrink tubing around it, to make it grip the accessory bar better.
I made my own ball mount from a 1" threaded ball from McMaster Carr, a short piece of 1/2" aluminum tubing, and a 1/4"-20 bolt. You could skip this step and just buy a RAM ball with a threaded end and screw it into the aluminum base (you'll need to tap a hole in the aluminum to match the RAM ball).
It sounds like more work than it actually is.