Fighting Blanche, Candela, and Spark. Open up your Nearby list, then go the Battle tab at the top to challenge them. Today, you'll be able to fight them 5 times, for a guaranteed Sinnoh Stone each time. Just do the Great league and you'll be able to churn through it pretty quickly.
You can also battle against other trainers. As with the team leaders - 5 battles, 5 stones.
