Salt from the coast...

[adilu]

So now that I have built a house on a small river island, I've started the advanced adventures quests. "Salt from the coast" seemed easy enough. I had enough stuff to barter for salt, since I was lucky enough to bring down a Njerpez that I stumbled upon randomly in a not-so-far-east region. Anyway, I live close to the western coast, so I took food with me that would last for only a couple of days. However, when I arrived at the archipelago and visited pretty much every village and settlement, there was not even a pinch of salt to be found! Even the food that the islanders hoarded was not salted. It was either dried or roasted. It was almost like salt was unknown to them. So I said "oh well" and went back home after more than a week and almost starving. A few days later, I actually needed salt so I decided to get one from the Driik metropolis a few kilometers south of my settlement. Guess what? The quest was completed!

So apparently you don't need to get the salt directly from the Islanders themselves. It works as long as you get a bag of salt by trading. I remember in earlier versions in Islander towns you could always find nets and bags of salt. In my recent games they appear to be very poor and underdeveloped with very little stuff to trade.

[JP_Finn]

Coast means on the shores of main land. And although I’m not on my computer at the moment, I recall Driik mentioned in some of the quest texts.
Driik is the only source of salt in-game.
(I suggested couple years ago that foreign traders would carry various size salt bags too)

[Matti-patti]

You can occasionally find salt in Sartola (just bought a bag there) and maybe Reemi but don't quote me on the latter (couldn't find any in quick verification attempt).

[Plotinus]

I also remember finding salt in Reemi, but rarely, and probably Sartola too.

Also I have more luck in Driik on the first time I visit a village, if I visit the same village again too soon, the salt is likely to be gone  (this is a normal game mechanic that the food items change from visit to visit, and also that expensive items are more likely to be generated after a long absence.