What other cut of meat can replace pork shank?
You could probably get away with pork shoulder, although you will want to consider that the texture may not be what you want -- it may either have too much fat and connective tissue, or it may fall apart too much, depending on how long you braise it for. You will probably have to alter your cooking times, I'd imagine.

Pork hocks are somewhat similar in texture to the shank, so that a possibility.  Though you will probably run into the same availability problems.

It may take a small amount of modification, but your best bet may be to go with country-style ribs. They are tender and flavorful when braised, and are readily available in any grocery store.