Spent a few hours looking for tar pits and nothing. Supposedly you can find them in unexplored areas of the plains. Problem is if you sailed past it in a boat that area is considered explored.
Guess what I did while waiting for a new boss. I went sailing to find the Mistlands. Built a new base with a portal right next to it for a quick start on the boss campaign. My map is toast for tar and onions.

Will probably switch to a new world just for tar and onion seeds.