Welcome to minis and welcome to driving!
Is that your mini in your avatar? BEAUTIFUL!!
You are also in a state that has Amish population, so you have some access to different cart builders and harness makers - can get something in stock/off the shelf or completely custom.
Harness:
There are in-expensive bio/beta-thane harness and top of the line bio/beta-thane harness that is made for CDE/marathon competitions that is nearly indestructible. I have some that fall in between. I deal with an Amish harness company out of OH that makes harness and specializes in bio/beta. Bio is the shiny type and beta is the flat type and comes in different widths, lengths, weights. They even make a type now that has bubbles on it - great for hand holds! I have both inexpensive harness and have done upgrades that raise the quality of my harness. I can get stock/off the shelf and it fits most our our ponies/minis or I can get custom pieces in sizes to fit my guys. You can do a completely custom harness both thru the ones I have mentioned below or thru the company that I go thru. I have both pleasure harness and also work style with collar and hames in beta and bio. The bio - I don't want any more of it. It's pretty - like patent leather shoes. For everyday driving, it's not as durable and gets scratched up.
You can clean/shine all of it with Pledge furniture spray and they now have bio/beta cleaning/shining products. I LOVE that I leave it on my sweaty pony and wash it off while hosing off the pony. Or you can sling it over an outdoor fence, harness rack and wash it off that way.
Names of companies that have the bio/beta for competitions - Zilco (actual name of harness type), Ideal Harness (company that has different types), Country Carriages (amish made harness - not sure who), Chimacum Tack (Comfy Fit Harness), Black Horse (Comfy Fit Harness), Herrons Tack (carries Comfy Fit). Comfy Fit is made by Amish - not sure where.
Ozark Mtn Mini tack, Star Lake Tack & Prime Design are all Miniature Horse Tack companies that also carry pleasure type beta-thane harness - both beginner/everyday and show quality. They have online catalogs and print catalogs. Some say that the "saddle"/surcingles don't conform to the horse - but the higher quality harness is combined and has some leather on it and works every bit as nicely as a leather saddle/surcingle.
Carts:
There are several companies that have both show carts and also ez entry carts. A Meadowbrook is very pretty and great for showing on grass at a CDE/ADA shows, but can get way too heavy especially if you have small minis (I have different heights of Shetlands - some are w/I the mini B height sizes and I am a LARGE woman) on trails and in show arenas with dirt/sand footing.
The same Amish company that I purchase all my harness thru also has different types of carts with options - some would be both local show and even AMHR/ASPC show quality. When you get to the Regionals/Nationals everyone seems to use either Jerald or Graber type carts. A Silver Penny and Sunrise Pony Farm both have different types of carts that are built by Amish. Be careful with the stock EZ entries that have the light weight frames and bike wheels - they will fold up with any kind of torque while pleasure, show and training driving...
There are a lot of choices out there. Yes, you can use a bio/betathane harness and ez entry cart at locals shows... There are many other companies that have carts, wagons and harness. I wish my Links page was working on our website - I have over 50 harness/carriage links on it!