Shimano SM-AD11 Front Derailleur Clamp $16.95
If you own a braze-on front derailleur and then purchase a frameset that requires a clamp-on front derailleur, this is what you need so you can keep using the derailleur you already own. It transforms a braze-on front derailleur into a clamp-on style in a matter of moments. You'll need to choose one of three sizes -- 28.6mm, 31.8mm, or 34.9mm (aka 1-1/8", 1-1/4", and 1-3/8" respectively).
Two notes of interest: These clamps are 100% compatible with both Shimano and Campagnolo front derailleurs. Also, the 28.6mm clamp isn't made by Shimano. Shimano doesn't manufacture a 28.6mm clamp. Rather, it's made by the highly regarded company, Problem Solvers.