Adventurer's Suit 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 9 and survives 30 uses; every crafting attempt spends one use.
Fleta: Buy a pattern from Fleta while she wanders Sen Mag Plains near the crossroads. (27,000 gold). She only appears at set morning, afternoon, and evening windows.
Materials
Expect about 3 attempts per finished item (40% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.
Each attempt uses
2× Fine Fabric — weaving on a loom, low rank
2× Fine Silk — weaving on a loom, low-mid rank
2× Braid — low-rank Weaving skill output
Finishing (once per item)
1× Fine Finishing Thread — Buy directly from any general goods NPC shop.