After using up all my Kraken jugs, I'm asked to use Tonics to restore my attack.
I look at my item's list and I don't see anything with this name.

I'm not sure how many I have left and what the effect is.
But after spamming 10 or so, I then realized that it restores stamina aswell so there goes a lot of level ups lol.

What is this Tonic and where can I find it?

Tonics are powerful items that fully restore your STA, ATK, and DEF. You were garunteed one after completing the tutorial, and it was the basic reward during the Buccaneer Rosa raid event. I believe Aeria has also distributed some as compensation for various bugs or so. Otherwise, you buy them from the shop with gold. I have no idea how you acquired so many tonics if you didn't buy them.