If you go with the "wheelchair" or "mobility chair" motor route (and as already suggested, for this application, I would) - you're going to find a couple of things:
1) The motors are expensive
2) The motors are expensive to interface with
First off is finding the motors. Your best bet is Ebay or Craigslist. On Ebay, for the motors only, look up keywords like "mobility", "chair", "wheelchair", "motor(s)", "scooter" and of course combinations thereof. Another good word to use is "transaxle", but be careful that you are getting a dual-motor differential drive transaxle that has two motors in it, and not a single motor transaxle with a differential (the first is used for chairs like the Hoveround, the second for chairs/scooters like the 3-wheel Rascal). These things weigh a lot, around 30-40 pounds for the motors and gearboxes, more with wheels; if you can get the motor controller with the motors, and it is in good shape, then get it.
On Craigslist, you are likely to only be able to find the complete chair; generally, the way it goes is that people get a chair thru Medicare, then pass away, and their family has to donate it or sell it - sometimes you can find them cheap, sometimes not, but generally they are waaay cheaper used than if you bought "retail" from a medical supply store (without Medicare). You can also find complete chairs on Ebay, too. I have also seen complete chairs at Goodwill (rare, though).
As noted, if you can get the motor controller (and it is in working shape), you want to get it. Otherwise, you are going to have to shop around for a suitably sized controller for your motors (after figuring out what the motor specs are - which may involve some phone calls, either to the manufacturer of the motor/transaxle, chair, or a repair shop who may know). Controllers are not cheap - but if you have to buy one, look into controllers for "battle robotics" platforms. There are also custom chair controllers out there designed for the mobility chair "modding" community (yes, such a group of people exist).
Finally - an option to consider (dubious, though, I'll admit) is to get the gearboxes, wheels (or at least the hubs), and motors from a dual-gearboxed 12 volt PowerWheels ride-on toy. Sometimes you can find these cheap on Craigslist, Goodwill, yard sales - sometimes people are looking just to throw them away. If you can rebuild the drive system for your custom use, they'll easily haul 200 lbs (though the motors are noisy like you wouldn't believe - unlike powerchair motors, which are designed for smooth and quiet operation).