I lost small plastic "collar" part on the front v brake which goes around the spring and onto the post. Does anybody know where I can get another one?
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:
- 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.