1. Mission Drops: Certain stages in the main story, events, and side stories have a chance to drop Skill Experience materials upon completion. The type and rarity of materials obtained depend on the stage difficulty and your account level.
2. Training Grounds: Training Grounds are daily stages specifically designed for farming Skill Experience materials. The higher the Training Grounds Level, the better the quality and quantity of Skill Experience materials you can get.
3. Event Shops: During events, event shops often feature Skill Experience materials at limited quantities. You can exchange event currencies earned from event stages to purchase these materials.
4. Base Skills: Some Operations have base skills that provide a chance to obtain Skill Experience materials when deployed into the base facility. Upgrading these base skills increases the chances and rarity of material drops.
5. Quests and Rewards: Various quest lines and achievements may occasionally reward players with Skill Experience materials upon completion or milestone reached.
Remember to use different types of Skill Experience materials depending on the rarity of the Operator's skill to level it up effectively.