Facebook market place, Craigslist are the cheapest.
Ebay, set the distance parameters on the side.
"Play It Again" used sporting goods stores.
Call / Email around to gym / fitness suppliers in your state. Many maintain a stock of used machines and plates from gym buy outs.
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Figure $.50 - 1 /lb for decent stuff locally. Look for Ivanko, York, Iron Grip, Cemco, Troy, Intek as first choices*.
*You might see Rogue, Eleiko, etc.... too but they'll be expensive and/or rare on the used market.
