Snagging a bolt off an old brake is probably the best bet.
If you are using an older frame from the 70's or early 80's, it may have had 27" wheels(630mm). To run 700c wheels(622mm) you may need long reach brakes like the Tektro R556/Rivendell Silvers sidepull brakes or the Dia Compe Mod 750's. Heck, on some old bikes you'll need either of these even if you use 27" wheels.
Loosescrews.com is a good place to find all sorts of hard to find or even hard to describe parts. If you aren't sure about something, give them a call.