Yeah, if you go watch the amish take a horse that was being broke to drive and it wanted to lay down or not pull. Pretty much what they do is hook it inbetween a team of draft horses and whether it wants to go or not, it gets forced to walk and if it fall down most of the time they will drag the horse a little ways and the let it get up. One or two days of this is all it takes to break a driving horse of bad habits...not that I agree, I just know this is what our old nieghbor did when people brought him a horse.
As far as finding someone who trains horses to drive, call around local horse training stables. Most of them know what needs to be done to teach one to drive.
The best option and cheapest, other than you having to buy a cart and harness, learn how to do it yourself. If you have ever trained a horse to ride, then driving is not to much different. You still line drive them from the ground and make sure they understand this before you ever even think of trying the cart. The biggest thing is getting them use to the shafts without them getting themselves tangled up. If you would be interested I can write a really detailed description on how to train a horse to drive (I train minis for everyone in this area). Then you can try to find someone to help you out, It is best with three people total when teaching one to drive, and teach them yourself. Other than using this as a choice, find a horse stable somewhere and I am sure at least one of them knows how to teach a driving horse.
You know how to make a miniature horse even smaller? Leave them in the dryer a little longer!
"Don't ever regret something that once made you smile"