IF you at all crafty - you can make a harness yourself. I even have two that I made out of RECYCLED, braided HAY STRING that I made originally in 2009/2010. I'm still using parts of it, other parts have been replaced as necessary. You can make a harness out of nylon, 550 paracord, haystring, bio/beta thane or if able to do leather work - you can do it that way. You can braid, stitch, rivet, screw or use conway buckles for your straps. Hand stitching is the most time consuming part also requiring specialized tools to make it easier to do (stitching horse of some type, or a vice mounted to some type of sturdy bench, needles, awl and punches of different sizes). There is a book out there for making your own harness out of leather. Not sure if it has the measurements for a mini sized harness - I haven't had the chance to look at it... I've been told it's a good one if you choose to make or repair your own harness.
Some of our hay string and paracord -
Braided Tack
The main part of a harness that is hard to build yourself is a good surcingle/gig saddle - especially with a tree. The weight of you and your cart is at times carried on that piece - the better one you have, the more comfortable your horse will be. BUT for training purposes, I started most of my current driving ponies with a simple Weaver training surcingle that cost all of $30... I got enough use out of two of them in the first two years that I wore them out. Got a 3rd and now need another one or two...
My very first breast collar - made for me by an Arab/SB/Hackney pony trainer - was made from a used western pony girth covered in a sock and had the crown from a used horse size halter riveted to it for an adjustable neck strap. The traces were made from military straps that cost nothing - they'd been thrown out by a military unit and he'd gotten them. My only cost was the snaps. He must have used good ones - they still work and they are 17 years old this year! That first harness didn't use breeching. I had a training surcingle that i purchased with a mare that I bought - it had the back strap and crupper on it. I've used it for the basic measurements for our larger Shetlands - had to make them much smaller for the mini sized ponies.
Some of the Mini websites carry a harness surcingle or gig saddle OR can get you one if you ask. Our website used to have many resources - but the Links page is currently out of commission. There are also Amish stores that can make parts for you - you don't have to have all of it at one time if you can't afford it all to start training. Breeching and crupper are needed once you start hitching, imo. I will share some resources if you are so inclined - privately.
I have an Amish shop in OH that I deal with a lot - they can repair harness that I currently have (if I can't or don't have the time). A friend called me from a Horse Rescue "yard sale" - there was a mini sized gig saddle for $10 (NICE ONE with brass hardware!!) with a backstrap and breeching but no crupper. Two hours later, they still had it and I bought it. Called the shop in OH and had them make the pieces I needed to match it (brass hardware) - with simple conway buckles. Inexpensive that way. 2 years later - I've replaced some of it with stitched straps and proper buckles plus the girth straps on the "saddle" had worn out and they replaced those. Now I just need to do the shaped breast collar with buckle in traces...