Yes, you can buy them, but I can garruntee the parks dont get them from some on-line store. Even so, at even a few dollars a ring, several thousand add up very quickly, particually if they are constantly replacing them.
Here is where you can find a list of operating brass ring machines:
http://www.nca-usa.org/NCAcensus.html
Also, when you WRITE something off on you tax claim, it is only a percentage of the actual value that you get back.