They're called e-clips.
Might be able to find one at a hardware store that's the right size. Or write to DSS and ask for another. They're the best way to work with those pins, or you can use a small nut and bolt. With the stock pins, you can sometimes get away with reusing them -- they flare at the end once they've been put through the holes, and after tapping them out sometimes they'll still work.
Best way to put them on is... carefully. Needlenose pliers work, pressing them on carefully.