See if you can get hold of any local mobility centres around your area and ask if they have any old motors/wheels, or if they can tell you where their old chairs go. They might have a particular scrappy they go to or removal company. Failing that, eBay is pretty much your best bet. You'll rarely get the motors/wheels on their own from there, you usually have to get the whole chair, most of which are collection only.

Scooter motors are indeed another option, but adds a bit of complexity with chain/belt reductions and making a frame for all that. The wheelchair motors will be a much easier bolt-in solution.