Mana Thistle requires a herbalist to be level 370 to gather and can be found at Isle of Quel’Danas. You may need a flying mount to reach other areas to collect them efficiently. It appears to grow in pairs, so whenever you see one Mana Thistle keep your eyes out for the other one because it should not be far away. Each node will only give you 1 or 2 Mana Thistle as it comes in a pair of nodes. Gathering Mana Thistle will restore the 500 to 3500 mana.
See below for where to find nodes of Mana Thistle and free mana restoration.