It's interesting that this subject has been raised. I had been missing the o-rings to the Campy brakes on my Motobecane for a number of years, but didn't do anything about it.
Recently, I went down to the local hardware store and picked up a couple of replacement rings that fit fine. I can't remember the exact size. Don't know why I hadn't thought of it sooner.
Dan Wyman
Oceanside, CA