Put your location in your profile, and we'll have a better idea of what your local hobby shop is!! Some hobby shops will be very helpful with control line stuff, others less so.
What Larry said. Add Brodak's to the list:
http://www.brodak.com. They have lots of stuff.
You probably want a three inch bellcrank. Don't even
think about using a little bitty 1/2-A bellcrank for the job!
Be sure to check back here if you get stuck again.
Do the instructions show how the bell crank should be mounted?