You can buy them in the shop with the coins you get for doing the Island Quests and Exploration, or they are a rare drop. The shop has a tab for them.
For the first upgrade I got 3 drops and had to buy the last two with coins I got off the islands as they're different to standard coins but I got most of what I needed exploring the first two chains of islands
Later upgrades look more dependant on refarming those islands to get coins for upgrades though
For the first upgrade I got 3 drops and had to buy the last two with coins I got off the islands as they're different to standard coins but I got most of what I needed exploring the first two chains of islands
Later upgrades look more dependant on refarming those islands to get coins for upgrades though