A lot of players are having the same issue, something about the latest update seems to have made it so the golden coconut buying option isn't getting added to the island trader, or for some players who already had it before the update, it got removed from the island trader shop when they updated.
There have been reports that in general, sellers that are supposed to get items added to their stock based on certain conditions also have not been working since the update, so that's something to be aware of if you're having trouble getting access to any similar items.
For now we'll just have to hope to find more golden coconuts in the trees...
