You could try all the local scrappies eg Wirral Spares, Green Lane etc and you might be lucky, I don't think they normally take them off so you might be reliant on them having a car in at the time.
Facebook market place.
Try a wanted on gumtree, facebook etc
Ebay - about £50 second hand, £90 new.
Be aware that if any part of a towbar is fitted, the electrics are MOT-able and have to be present and fully working also the towbar itself is subject to inspection whether whole or part.
Also you have to inform most insurance companies as it is usually classed as a modification.