Cores Thief Suit (M) is crafted with Tailoring in Mabinogi (clothes). This page lists everything needed to make it, checked against the game as it exists today. The interactive planner lives in the Tailoring Companion.
How to get the sewing pattern
The sewing pattern is rank 3 and survives 30 uses; every crafting attempt spends one use.
Walter: Buy it from Walter's General Shop in Dunbarton. (112,500 gold). No special requirements, just gold.
Effie: Buy tailoring manuals directly from Effie's shop at Qilla Base Camp in Port Qilla (112,500 gold). Standard gold purchase, no special currency needed
Granites: Buy a pattern from Granites's general store in southwest Filia. (112,500 gold). Elf-allied humans get a discount, other races pay full price.
Solea Chest: Open a treasure chest in the Solea caves using a Treasure Chest Key.. Requires a Treasure Chest Key, a rare drop from monsters in Solea.
Prairie Dragon: Defeat the Prairie Dragon that spawns northeast of Maiz Prairie in Rano.. Spawns only in two daily windows, mid afternoon and early evening, and can be fought as an open field raid.
Materials
Expect about 10 attempts per finished item (10% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.
Each attempt uses
1× Finest Fabric — weaving on a loom, higher rank
1× Finest Silk — weaving on a loom, higher rank
1× Finest Leather Strap — weaving rank 3-1 on a spinning wheel
Finishing (once per item)
1× Finest Finishing Thread — Buy directly from any general goods NPC shop.