you should be able to pick up a pack of double end threaded 4-40 rod at most hobby shops. Look in the airplane section, probably with the Dubro parts. Honestly I make all my bends with a pair of pliers, never had a problem.
you can also get them in a fully threaded version. If you need your custom likage shorter than what's in the pack pick up a rod end adapter (by Dubro as well) It's a piece of brass that is threaded on one end and hollow on the other, you simply slide the hollow end over the end of your 4-40 rod and solder. Works nicely, I have two custom linkages on my T-Maxx made in this fashon.