They do come up on ebay occasionally, you have to keep your eyes peeled - watch out for "rear door trim". Last n/s one sold about a week ago, £7 plus post.
As VicW says, other obvious answer is dismantlers, or if you are feeling particularly flush, Honda dealer.
Note that secondhand ones quite often have broken clips and have to be bonded on, best stuff to stick them on permanently is polyurethane body sealer.