Used hay rakes can be purchased at dealers that sell new and used farm equipment. Craigslist.org also has used equipment listed by owner and sometimes dealers as well. Bulletin boards at feed and farm supply stores are another great source for used equipment.
Auction sites such as Ebay.com have listings for used hay rakes. It can be helpful to post a wanted ad on Craigslist or used farming equipment forums, letting others know what type of hay rake is needed. Smaller rakes for odd brands of tractors can be harder to find and often need to be purchased new. Visiting sites dedicated to a particular brand can lead to positive results.