A Substitute for beef shanks?
Oxtail would be your most likely substitute, but you have indicated you cannot obtain it.

The next best choice (of those widely available) is probably chuck, which is the shoulder muscle, and is well worked and flavored.   It requires low and slow cooking for best results, much like shanks and tail do.



Some stores&mdash;even the chain grocery stores&mdash;will special order for you if you ask them to, so you might be able to get shank.  Depending on where you live, you may also try butchers at farmers' markets, Dutch markets, or similar.