Bolt of Silk Cloth is one of several types of cloth that a tailor can use to make different types of armor. It is crafted, looted and sold by NPCs.
Bag Space in WoW – Owlaf's Blog
Rich Purple Silk Shirt - Spell - WotLK Classic
Tailoring Guide 1-525 - Cataclysm Classic - Warcraft Tavern
WotLK Tailoring Guide: Turbo-Boost Leveling from 1 to 450
WoW Classic Cloth Farming Guide - Silk Cloth, Mageweave Cloth, Runecloth, Felcloth - Wowhead
Cloth Farming in WoW Classic FRESH - Warcraft's Season of Mastery
Cloth Farming in Classic WoW
WoW Classic Cloth Farming Guide - Silk Cloth, Mageweave Cloth, Runecloth, Felcloth - Wowhead
Some WoD Crafted Armor Sets and Weapons Unobtainable in Shadowlands - Blizzard Reply - Wowhead News
Robes of Arcana, WoWWiki
One Hour Grind in Alterac Mountains--120 Silk Cloth Per Hour--Gold Farming in WoW Classic
One Hour Grind in Alterac Mountains--120 Silk Cloth Per Hour--Gold Farming in WoW Classic
Dark Silk Shirt - Spell - World of Warcraft