You won’t find every ingredient you need for a recipe in a general store or town. Most of the time, you need to spend browsing through the dungeons and attempting to locate anything of value, and locating glue in Rune Factory 4 could be a little tricky. However, once you reach a point when you can access it, you can find it far more frequently.
The glue starts to become more frequent when you gain access to the Yokmir Cave, which is a location you can access by going through the Yokmir Forest. The location doesn’t become apparent to you until you reach a certain point in the plot, but you can find it before this point. It’s in the second area of the Yokmir Forest, at the northern cliff. When you approach the cliff a small, red triangle appears you can interact with to jump off.
You won’t freely find the item on every creature in the dungeon. You primarily find it when taking down High Orcs. These are the monsters with clubs, slightly similar to the ones you fought against in the Yokmir Forest, the first dungeon you encountered.
The enemies are relatively strong in the Yokmir Cave, so you may want to save right before you enter. Take these creatures down reasonably steadily, and you should find yourself collecting glue pretty regularly. Glue won’t show up as much before this point, so don’t worry about if you can’t seem to find any until you enter this cave. After this point, glue should become a more regularly obtained ingredient.
You can freely farm High Orcs for glue whenever you finish the Yokmir Cave.
