I assume you just mean the metal band that goes around the handlebar, and the shifter screws into. These are different for STI levers than they are for traditional brake levers, but it's still the kind of part where there are lots floating around. I'd expect a bike shop to have them in a bin somewhere. I couldn't find them on loose screws' website, although they're still may have them. I'd just say check a bike shop for a part like that though.