I'm pretty sure Northern Tool and places like that have them. Dealers, especially in dairy areas, probably have them setting in their show rooms.
A couple of comments about them: Since it takes time to put them on, they aren't the best for emergency backup supply to a house. They are also fairly expensive since most are matched to typical PTO and tend to put out pretty big KW's. I'm not sure it's desirable to put a generator that could be driven by a 12hp-gas engine on a 24hp tractor. It seems like a good idea to think through the power requirements.
If you get one, it's a good idea to get one that has a frequency meter. Electric motors are quite sensitive to the AC line frequency. Running a tractor slow produces a low frequency that can burn out electric motors. At least with my hand throttle, it's hard to set the tach right on the 540 pto mark, and I wouldn't be sure how a pto speed error translates into AC frequency error. However, somebody did say that plugging an electric clock with a second hand into the generator supply could check frequency by comparing the seconds to a battery operated clock.
The tractor should be 'over-powered' for the generator. If it's possible for an electrical load to lug the engine, then the AC frequency falls, and electric motors burn out. With an over-powered tractor, the generator mains would pop before the engine lugs.
If the generator is for a new application, what the generator will run should be considered. There are quite a few codes most places that have to be met for a generator to be hooked to anything also supplied by the electric utility. For example here codes require that generator supplies be wired through transfer switches that have 3-point breakers (two hots plus the neutral) and cannot physically have both the utility and the generator supply on at the same time.
The code requirements can be a bit of trouble and can easily turn a big generator project into a smaller one.