Currently obtaining material is a lot of pain, so I have two proposals for how to resolve the issue:

1. After ranking up, unlock an area to explore freely with respawnable enemies (exploring ends if the players dies or requests extraction), after beating the game all areas are unlocked.

2. Dedicated NPC for gathering materials and reselling them, working similarly to Dust: An Elysian Tail, in that you have to sell them the material for them to add to their list.