I’ve been trying to find a used full-size/warmblood size harness, but all I’ve been able to find online are new ones from sites like Smuckers, Driving Essentials, etc, and I REALLY don’t want to pay the “brand new” price. Are there any good sites for classified ads? Anywhere in the USA is fine.
Try putting a want ad on the CD-L. There are usually folks there trying to sell things.
I did that, but no one had quite what I was looking for (or priced it the same as a new harness!). 
Try www.draftsforsale.com and peruse the tack and harness. It is a lot of draft harness but I have seen pony and horse harness too. You can also put up an add on their site, I did that and got a very nice set of used team harness for $200!! Can’t beat that. Have you tried doing a google search?
When I was looking for harness I Googled “used driving harness pair pony” and “used driving harness for sale”. I got a great buy on a fairly new Yonie’s pony pair harness that fit the girls perfectly
Then when I wanted a pair harness for my warmbloods I did the same thing “used driving harness for sale horse”. I went through all of the ads and got another Yonie’s harness. This one can convert to two singles and has the extra pieces to do so as well as an extra pair of traces. One side was put on a horse 3 times; the other side had never been used. I got it for $800 under retail 
There are lots of good buys out there but you have to be willing to do some legwork to find them. Good luck!
Claudette Robinson at Country Carriages in NC frequently has used harnesses for sale. At retail dealers, it will all epend on what might have been recently traded in or that they picked up as a deal.
The other most likely places are classifieds in local club newsletters
Sometimes you can find things on ebay (havent looked myself)
but it is sporadic
There are several very good Amish harness makers who will make a new leather harness - good quality for around the same price as used
Best of Luck in finding what you want
There was an ad today on CD-L from the folks at Advanced Equine. They are selling their horse sized harness.
Harness and best buy on bits
There’s a place in Indiana, from which I have bought a number of things i have been very happy with–most notably driving bits for my 5 inch mouthed 13hh pony–but I may mangle the spelling here: Shipshewanna Harness. They are fast, friendly and reasonable–as I am not going to pay a hundred dollars or better for a bit.
Ther have all manner of harness, parts, et cetera. Sometimes I just like to browse the site and wish.
Harness and best buy on bits
There’s a place in Indiana, from which I have bought a number of things I have been very happy with–most notably driving bits for my 5 inch mouthed 13hh pony–but I may mangle the spelling here: Shipshewanna Harness. They are fast, friendly and reasonable–as I am not going to pay a hundred dollars or better for a bit.
Ther have all manner of harness, parts, et cetera. Sometimes I just like to browse the site and wish.
Too bad. I’m reluctantly selling my cob sized light driving harness. It was for a 14 hand 900 pound arab/appy on all the tightest buckles, but I doubt could fit a big horse even if “let out”. You only get 3" more at each buckle closure.
Where to look? Well, I posted my ad:
Online on equine discussion forum
Local (& surrounding area) Craigslist
Flyers in horse barns at State Fair
Mailed out to area tack shops
I won’t put it on ebay, hate their policies. But I boxed & weighed it and at 20 pounds will only cost around $30 to mail, Fed Ex may be even cheaper. If I don’t sell it soon, I’ll post it on equine.com or horsetopia. They charge for ads though and I’m trying not to spend right now.
Good luck! I miss driving.
