Caterpile Silk

In Bionis Leg, dropped from Royal Caterpile. They are found north of the bridge in Wind Cave, which can be quickly reached by Skipping to Spiral Valley then jumping off the bridge. Level 60+ can easily survive the fall.

Alternatively, Slyviane in Colony 9 can be traded for them when she is given a very expensive item. Also can be traded with Dorothy in Colony 9 (Affinity lv.5 required).