Tembea Industrial area. There's a big shop at Dunga Road/Bandari Road called CMS (Commercial Motor Spares), 0728-866 688, branded in red and white colors if I recall well.They'll get the part number for you as well as the genuine part itself. If they do not have it, the can arrange to get it for you. And yes, their prices can often be over 5k cheaper than TK, depending on what you want. Directly across them is another smaller shop that also retails genuine Toyota parts but whose name escapes me at the moment. TK themselves have a shop along Kirinyaga Road.
All the best.