How do I get ingots?

Extract ore from the Mine (unlocks at level 21), build Foundries, and smelt ore into ingots.
Play at the House of Luck to win them as prizes.
Open chests in the Mine and in your friends' towns.
Open golden hot-air balloons in your town and your friends' towns.
Send fully-loaded planes.
Race in regattas for the chance of earning ingots as rewards.
You can always use Township Cash to buy the ingots you need.