It`s called a door sill garnish. Dont think you can buy just one comes in a kit of 2 from Honda.Could try ebay.
Maybe check under the trim for any signs of rust on the actual sill as these parts dont usually rust and it might be creeping onto from the sill.
I've used 24/7spares a few times, it acts on behalf of a load of yards and they contact you individually if they have the part. Don't give your number though, just your email or your phone won't stop lol