Riding 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 8 and survives 30 uses; every crafting attempt spends one use.
Nora: Buy it from Nora at the inn in Tir Chonaill. (73,000 gold). No special requirement, just enough gold.
Galvin: Buy a pattern from Galvin's shop under the Observatory in Emain Macha. (73,000 gold). None beyond having enough gold.
Granites: Buy a pattern from Granites's general store in southwest Filia. (73,000 gold). Elf-allied humans get a discount, other races pay full price.
Materials
Expect about 6 attempts per finished item (19% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.
Each attempt uses
4× Fine Silk — weaving on a loom, low-mid rank
2× Tough String — weaving Thick Thread Balls
2× Tough Thread — weaving Thin Thread Balls
Finishing (once per item)
1× Fine Finishing Thread — Buy directly from any general goods NPC shop.