There are tons of spotted mammoth donkeys out there. The trick is finding one that's trained for anything more than breeding everything in sight, and isn't owned by idiots.
There's a big breeder of spotted, gaited donkeys and mules not far from here, but their website is plastered with things like, "here's a video of us riding the two year olds, look how gentle they are!"
http://www.boxerworld.com/rescue/excuses/