There is route one, which does your basic wethepeople, mongoose, hoffman bmx's - not much choice though. Thats one briggate near pizza hut.
Halfords - 10 mins walk behind the station, usually halfords stuff. It is a bikehut.
Woodrup Cyles - Kirkstall Road (get a bus from headrow) its just slightly further down from Vue, Hollywood Bowl, Virgin Active, Pizza Hut retail park place. On the left. Near my house. Not actually been there in ages though, not sure on the choice.
Mike