As far as custom rotors, we harassed a bunch of local laser-cutters till we found one that would make them for free for us. Now all we do is send them a CAD file and they send us rotors, it's great!
As for floating rotors, we make our own floater nub thingies. It's not terribly hard other than the fact that they need to be tight-tolerance so your rotor doesn't flop around.
As for bikes that come with floating rotors, from what I've seen they're mostly late-model sport bikes...meaning they will almost certainly be bigger than will fit inside a 13" wheel.
The rotors and floaters are relatively easy to design; in fact I imagine that in the time it would take to search for a motorcycle rotor that would work you could probably design one and also find a laser cutter to make them for you.