https://boulderbicycle.bike/2012-Cam...ice-price.html
$23 plus shipping. Only fits if the one you need is a front, for external (not recessed) nut. Or it sounds like Jim Merz has just the one you need, and his asking prices are always reasonable.
But the OP didn't say what model or vintage of Campy brake, so if it's some more modern brake like a dual-pivot, all bets are off.
General rule: Pics or it didn't happen!