You won't find all the ingredients you need for a recipe in a general store or town. Most of the time you have to go through the dungeons and try to locate anything of value, and finding glue in Rune Factory 4 could be a bit tricky. However, once you reach a point where you can access it, you can find it much more frequently.
Glue starts to become more common when you access Yokmir Cave, which is a place you can access by walking through Yokmir Forest. The location doesn't appear to you until you reach a certain point in the plot, but you can find it before that point. It's in the second area of Yokmir Forest, at the north cliff. As you approach the cliff, a small red triangle appears that you can interact with to jump.
You won't freely find the item on all creatures in the dungeon. You find it mostly when deleting High Orcs. These are the monsters with clubs, slightly similar to the ones you fought in Yokmir Forest, the first dungeon you encountered.
Enemies are relatively strong in Yokmir Cave, so you may want to check in right before entering. Take out these creatures on a relatively regular basis and you should find yourself picking up glue on a fairly regular basis. Glue won't appear as much until this point, so don't worry if you can't seem to find any before entering this cave. After this point, the glue should become an ingredient obtained more regularly.
You can freely farm High Orcs for Glue every time you complete Yokmir Cave.
