I do think they gave away some Sky Whirl cages, because I know the guy who runs greatamericaparks.com has three of them, but generally rides will either be sold, moved within the chain, or sitting backstage. Sometimes parks will use ride parts for prizes at enthusiast events.
I think most rides for sale, from flats up to full coasters, usually end up on Ital International site (http://www.italintl.com). Not sure if Great America has ever listed anything out there, but I'd bet money back in the day Shockwave was listed there.
Six Flags generally sells off rides though International Rides Management, and only get listed online of they don't sell for a certain amount of time. As for parts SFGAm has a strict policy that all used rides parts must be destroyed before being disposed of basically without exception. Steven Wilson got his cages from PGA
Favorite Wood Coasters: The Voyage, Ravine Flyer II, Thunderhead, Balder Favorite Steel: Voltron Nevera, Steel Vengeance, Expedition GeForce, Olympia Looping Parks visited: 232, Coasters Ridden: Steel: 894, Wood: 179, Total: 1073