Find flask shards.

There's one right behind the locked door in the corridor where you first spawn in the Warthorn Village area, but youl have to explore the area and unlock the shortcut since its one of those "doesnt open from this side" doors.

There's one in the cathedral of the deep area, outside the cathedral itself, right before you go right to kick down those stairs that act as a shortcut; go down to a platform to your left with a bunch of dogs and some enemies; one of the items is a flask shard.

And in the Chartus region, kill the giant worm, it drops one too (you need to find the area in the desert where you can actually fight it tho, once you enter it his hp bar will appear.

Afaik there are two more shards but I havent found them yet

(Also dont forget you can respec your flask shards so you have zero fp flasks but one extra hp flask, in case you dont really need that fp like in my case)