You need a conclave with at least one essence and must have researched Arcane Mastery in the magic technology tree. Follow the improvement line that starts with Herbalist -> Apothecary -> Alchemist and ends with the Alchemy Lab, then you can buy the Iru Elixir within that city's shop.

don't worry, we all had that exact question once. It's not like the game clearly bumps you into it...

There's a potion of healing (forget the actual name) you get from an early on quest (the one involving butcherman) that automatically cures ALL injuries. Can't be bought, but might drop as loot.

You get the choice (after the Butcherman kills the kind old lady) of giving the potion to the Butcherman to get a Butcherman pet, or leave him and get a potion that cures ALL injuries on one hero.