Craigs list, just gotta look hard. I bought 500 pounds of 45 pound plates off someone. 40 bucks a plate instead of 90-100 a plate new.

I know in my area at least, there's an insane amount of Olympic weights at the scrap yards here. They were selling them for around $.43 a pound. I just happened upon it when I was setting up the home gym. I picked up over 700 pounds for around $300.
They needed a paint job, but the weights in the old gym I go to looks just as bad.

Try finding some gyms in your area that are closing or out of business. You may try a google search. In the past I have seen companies selling equipment that they have bought from closing gyms for re sale.

Craigslist is usually your best bet. Just keep checking. Otherwise shipping will kill the deal.