Some have used old brake disks for the rotors which are just about right for this job. Actually, old brake drums can be used, too.
This is a lot like a Hugh Piggott project, so you can check this out for inspiration:
http://www.scoraigwind.com/liselotte/index.htmhttp://www.scoraigwind.com/selfblade/index.htmhttp://www.scoraigwind.com/books/books.htmIt's a very old website, and some links are broken, but stick with it because it's worth it.
If you want to look for old DC motors or servo motors to re-purpose, you can look for old mobility scooters, wheelchairs, electric hand-trucks and shopping cart movers.
There are also motors of the right size in high-efficiency furnaces and variable speed air conditioning units. These are still newer things, so less supply of discarded stuff, yet.
If the scrappers don't take appliances, then probably the appliance stores do instead. This varies a lot from jurisdiction to jurisdiction. In mine, there are recycling regulations in place. Sometimes that means you can't get your hands on the old stuff once it's in the recycler's hands. But that doesn't mean the appliance store doesn't take a few old ones back, to help the customer make the sale, so they have lots of discards behind the stores, here.