I groomed dogs for 15 years, and never used anything but Andis' line of clippers; any of the super two speeds, or the cordless AGR work great on horses. I still have my first Andis 2 speed clipper, from when I first started in grooming, and have only put a few replacement parts, and a new housing on it, and it still works like a charm. I use my clippers to clip my goats too, and they work really well. They have alot of power, and are pretty reasonable; the cordless is a little more spendy, around 230$ for everything, but is well worth the investment.
I agree, you may be able to find these on e-bay, but there are plenty of dog grooming, and equine sights that will have them for reasonable prices as well.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."