What size harness are you looking for?
How much do you want to spend?
I like Smucker's brand I know the quality and it is a great harness, can be pricey even used. I did get one used, horse size for, $400 last fall, right place , right time.
You can try carriage driving classifieds on facebook. you need to ask to join.
There are harnesses for sale on there, and you can post that you are looking for one too.
I will keep my eyes open for one.
The best time to plant a tree was 20 years ago, the second best time is today.