I just replaced a broken one today. I went to lowes and bought a cheap aluminum extendable paint roller stick to use as a tiller handle. I cut the threaded portion off and drilled two holes to attach the yoke.
The handle adjusts from 4ft to 8ft simply by twisting it so I will be able to use it while trapezing. I don't know yet how this will hold up, but if it breaks this year or next I'm only out 10 bucks. It has a blue rubber handle that even matches my trampoline nicely.
I ended up on this thread because I too am looking for a good replacement handle for my tiller. Similarly to TPARRISH, my last handle was a paint-roller handle which was fairly easily extendable. Unfortunately though, on my last capsize my clever paint-roller handle bent so now it is completely useless as it is 'stuck' in the extended position (I was out on the wire at the time).
Bottom line - The paint-roller handle was a cheap and easy replacement tiller handle. Unfortunately, It was also far more fragile than I had anticipated.
Good luck everyone else!