eBay would be the most obvious place (often where computers sold from government auctions or business liquidations end up), but also e-waste recycling centers, actual auctions held by the companies themselves (this is where having a guy on the inside willing to give you a date of liquidation would be perfect), or just simple donations and giveaways that are "as-is".
Do note you can't take all machines that are being removed - in the US at least, computers bought with public money (most often schools), must be sent to e-waste or scrap reclamation due to compliance with government accounting mandates. There are exceptions to this (auctions), but those are usually never at schools or libraries.