I've had(and sold, since I'm pretty well out of breed showing)a couple of Lutkes--my everyday harness is Smuckers--I also have two other Smuckers for showing-a Pleasure and a Russet Deluxe(an absolutely GORGEOUS piece of work, for sale, BTW, as I have no vehicle it is correct to use with...). Have a pair harness made by someone no longer in business-very nice, but not 'top-of-the-line. My latest favorite harnesses are a single with deep V breast collar, and a pair harness, in betabiothane, from The Carriage House, in FL.
For suppliers who deal pretty much only in mini/pony harness, based on what I have seen, I would recommend Ozark Mt., Mini Express, or Star Lake--all seem to handle decent harness that isn't too expensive.Estate Supply has some nice-appearing harness, also-will cost a bit more, I believe. Ozark Mt. now offers a VERY nice looking ADS-type harness, too(which our "own" Dr. Pam had a hand in designing!-it is priced comparably to that of some of the higher end makers, though--with harness, you will find that you tend to get what you pay for-the REALLY good stuff costs some decent $$.