Where do I get this part for my v brake?
You're probably not going to find this part without cannibalizing a similar enough brake arm. 

The parts you can find replacements for without having a sacrificial arm for V-brakes (*) are:


Noodle
Boot
Mounting Bolt
Brake pads/cartridges


If you need to replace/repair any other part of the V-brake, you're better off (and likely going to be) buying a new one. 

(*) for most V-brakes; check the service manual for your particular model, though even if it is serviceable, you're unlikely to find the parts. 



That being said, you might be able to make a suitable replacement on your own for the part in question with a bit of PVC tubing or some other plastic tubing cut to size and shaped appropriately.