I had to replace the u-joints in my shaft and had the same problem. Ultimatly, I was able to get a set close (but not right) from Baum Hydraulics. Before calling them, take your old joints apart and mic them. They will need these dimensions. I found a set that was close, but the width was just a bit too much. I couldn't get the C clip back in. I ended up grinding a small amount of material off the cross below each cup and that did the trick. I don't use the tiller very much, but it's been togther and through several gardens now with no problems.
Remember guys: These tiller are really cheap, but they're worthless with out that driveshaft! If you are getting a tiller, make sure it comes with the correct shaft and the joints are tight!