Sometimes these pieces break or fall off. So, print one of your own with your 3-D printer. I, mean, really, it's a small part, and you bet your boots it's possible to print. So go on, make a few. They only take like 20 minutes each. And, when the one you printed out breaks, you put on another one from the same batch. How simple is that.
Somebody else made the exact same replacement part, but I thought that it mirrored the Traxxas part too much, and didn't take advantage of the 3-D printing capability. http://www.thingiverse.com/thing:92675
Also this may not be finished yet.
In the photo, V1 is on the top left and V2 is right next to the black Traxxas part. V1 had the problem that the groove for the spring was not deep enough. I removed the file for V1.
BTW, the vehicle in the other photo is a Rustler.
Edit on 3 April 2017: Added SCAD file, to make these in a variety of sizes. The customizer app should work, just the preview may not look correct.