Where would be the best place to look for heavy tires (400-600 lb range)? I'm in Atlanta so if anyone has any specific locations that'd be great, if not general suggestions of types of places to look would also be appreciated.
Call around to tire stores and mechanic shops and ask if they have any. Sometimes they'll give them to you as long as you take care of transporting them because they are a hassle to keep around if no one is buying them. Also, tractor or farm supply stores and trucking companies sometimes have them for sale. I got lucky and found one in a junkyard once too. It's been kind of like a scavenger hunt every time I've gotten a tire but they are out there.
I'm looking also in the DC area, Ill give that a try.
Look into commercial tire service centers. My local Town Tire has a commercial center and they give away the tires.
I know you said "specific" so I'm not trying to be a tool bag here. Just a general thought I guess. These tires should be free or close to it with a little bit of footwork considering you don't need anything that's functional apart from being heavy and round, so as soon as one of these heavy round things can no longer be used on a vehicle, it becomes a burden to the owner.
So, since the shops have to pay to dispose of them, they're usually happy to let you take them off their hands. I have a 50lber for dragging with a chain, and when I went to pick that up the guy walked me through his inventory of tires from small up to 350 lbs that I can take whenever I get enough room to store them.
I'm about 3 hrs from you in Aiken, SC.