Lesser Clarity Potion

Regenerates the Hero's mana by 100 over 30 seconds when used.

This concoction resembles a clarity potion, but is translucent rather than opaque. It functions like that potion, but for conferring a reduced benefit.

Source
These can be bought from Ancients of Wonders in Frozen Throne.

Malfurion, Tyrande, and Maiev can find one in Balancing the Scales, chapter 5 of the Frozen Throne sentinels campaign, Terror of the Tides.