This might help.
The mid-2011 model appears to be the latest, but I've heard that those that shipped with 10.7 may have had a firmware modification that might make booting to Snow difficult or impossible. I don't know enough about that to say anything definitive.
Also I understand that it is possible to run Snow Leopard from Snow Leopard Server ($20 from Apple phone sales) in a virtual environment on the new iMacs.
Here are some sites I've bookmarked over time that sell used Macs--at least some of them do. I don't have any personal experience with any of them, but they've come up at various times here or on other Mac boards. Check out the reviews for them. Personally, I'd be wary of buying a used computer on eBay. At least these should come with some kind of warranty and will have been inspected.