There are six ways to get potions:

1. Merge gems of the same colour
Different colours give different potions. For example, merging green gems together gives you Slow Motion potion, while merging yellow gives Double Gold.

2. Lock picking
You will sometimes earn potions while trying to open a dungeon door.

3. Spend orbs
You can buy potions any time during an attack by spending blue orbs.
4. Star Master
You can also obtain potions from time to time by completing the missions of the Star Master. Keep an eye on it!
5. Guild Armory
Visit your Guild Armory and spend some honor to claim the potions!
6. Lucky Thief
It is also possible to get some potions while having the Lucky Thief spell activated.
7. Collect them at the Bazaar

8. Receive as a reward in the Events