Glowberries become available at level 8. You can get them by chopping down Garland trees of various sizes on Home Island, as well as chopping down pink trees with blossoms on Faulkore Island, and by chopping down green trees with sparkles on Whisper Island.
You will need glowberries at the Smelter to make blue glue, which unlocks at level 17.
To learn more about resources and where to find them, check out our Resources section.