Where do all the tough old birds go? The first comment or answer that says "Alaska" will be promptly downvoted.

Seriously though, I keep hearing about the superiority of "stewing hens" for stocks and soups, and I'd really like to get my hands on a few. All those eggs we buy have to come from somewhere, I've got to assume that the hens that make our eggs are eventually turned into soup. I'm not asking for a specific source recommendation, just for clues to direct my search. 

Does anybody know where the old birds go?
They go into soup.  Or pot pies.  Or any of the other myriad industrially produced canned or frozen foods which require chicken stock or chicken flavoring and sometimes canned chicken meat.

The millions of cans of Campbell's have to come from somewhere :-)

For smaller home chicken farmers, they probably end up in the stock pot.  If you know people who keep backyard flocks, they may be willing to sell you their culls for a very reasonable price.