Sorry to disappoint you but all parts on CofA aircraft have to be replaced with the approval of a licensed engineer unless stated otherwise in the ANO as pilot approved. On 'Public', then everything has to be done by an LAE.
As for the parts themselves (even seat belts) they have to be that which the aircraft was originally certified with - there will be a part number in the appropriate Parts Manual.
Piper PA28 lap straps are by AmSafe and cost about £60 per seat. The inertia reel shoulder harnesses are by Pacific Scientific and can be overhauled at their European branch in Maidenhead, Berks.