Pure essence is an item always in demand due to its use in the Runecrafting skill. There are a few ways to get to the rune essence mine, but the best free-to-play method is via Aubury. He is found south of Varrock East Bank in his rune shop.
First, go to Aubury's Rune Shop, and right-click the teleport option on Aubury. Go to the closest rock and start mining. When you have a full inventory, go to the closest exit portal, and then run back to Varrock East Bank. Then, go back to Aubury and start again.
At 66 Magic, the best pay-to-play method is via the Wizards' Guild. The Yanille bank is next to the guild, so that makes the banking process easy.