f.i.g Bike Daikanyama thats where. I first spotted the Xtracycle Radish (pictured above) in their Daikanyama store but expect it is available at their Harajuku store also. I haven't had a chance to revisit f.i.g Bike and talk to the staff, but assume that as they carry the Xtracycle Radish they must also stock the Xtracycle FreeRadical and accessories.
f.i.g Bike Daikanyama is easily accessible from either Daikanyama or Ebisu stations. Check their homepage for access details.