I groom dogs and cats, and have for roughly 15 years now...I still have my original Super 2 Speed...the 'boxey' style one, and they still work really well! I like Andis over Oster, because they are so much quieter, they run cooler usually, and of course, blades are interchangeble. The only regular maintanance I have to do on my clippers is replacing the blade drive which is super easy; if you are showing, thus clipping, very regularly, you will want to make sure to change that driver every month or so, as that keeps the clipper running nice and smooth, and quietly, holds the blade on better, and keeps the blades working properly.
I also use my Andis' on my mare; she got a body clip this past winter using one of my dog clippers...took a bit longer, but they powered through her thick coat like it was nothin...it was the blades that didn't like dirty horse hair so much, hahaha!
So, yes, I definitely second the above post on Andis...I've gotten one of mine at a fleet supply store, in the live stock section, so you may be able to get one there; otherwise, look online on sites like petedge.com, groomers.com, etc...I think even horse sites have them, but I KNOW you can find them on a grooming site.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."