Thanks for that, no-one I talked to ingame seemed to know, everyone thought they were bought from a vendor. Mind you by the time I get to Lone Lands, I may not need those level 15 potions anymore, as I am level 23 now and just about to start that area. Roll on tier 3 potions I suppose!
The problem seems to be to find which areas they drop in. From what I am now told, they seem to drop in quest areas 7+levels higher than the potions they make. Which means that by the time you find them and start making the potions, you are actually needing the next tier potions. Weird system!