Ah, we got the harness from a local farm auction for about $50. We just lucked out and it fit him like a glove. ^_^; Sorry, that's not too helpful.
But I think American Saddlery makes similar ones for around $150 (don't quote me on that, I'm not sure), and there's Smucker's (at Smuckers Harness Shop: On-line Catalog Miniature Horse to Draft Horse Driving Harnesses and Supplies
) that makes them, too; if you just search for driving harnesses you'll get all kinds of results. But yeah, Bandit's somewhere from 12-13 hands (I've never truely measured him, I just guessed by comparing him to my mare).
If you check out that smucker's site I mentioned above, they have a page on how to measure for a harness, and that should help you figure out size, at any rate -- if your horse is around Bandit's, it ends up being an odd size somewhere between pony and horse.