First, since mini is just supposedly broke to drive, but you havne't seen it done with said pony... not that I'm doubting it's true, but it would be best to ground drive first, make sure he has that down. Then progress to cart as if you were training the horse... assuming he really is broke (I don't doubt that he is) this will be done quickly, but it's best to make sure ;)
Get a Frontier cart. Do a google search. But you can try to find one used. Check out Craigslist, contact a member of the local miniature and local driving clubs in your area to see if they know of any....
You'll need a harness, get one with breeching! For a good quality inexpesive harness, I recomend Zimmermans. You'll find their number at the page I"m about to send you a link to..:
http://www.cde4vse.com/links.htm
Stay safe and have fun! Driving is a blast ;)
jessi