I broke the washer pictured here in half. Clean in half. I was able to tighten down my nuts enough to ride, and my chainline is fine, but I'm starting to worry that it's an important part that I have to replace.



a) What is this called? Spacer, washer, transmogrifier, what?!
b) Should I replace it? I think I should.
c) Does anyone who works at a shop in Chicago know if their shop carries one? Is it a part a general bike shop would have?

Thanks for your help, everyone.