Derotain Mudsipper is a Thorium Brotherhood NPC in Gadgetzan, Tanaris. He gives the Imperial Plate quests: you hand in Thorium Bars and get the Imperial Plate plans. He sells nothing.
He is outside near the anvil and the forge, near the southern exit of Gadgetzan. (51.5, 28.6)
Tanaris
Derotain Mudsipper
Which quests you need for leveling and how many Thorium Bars they take is in the TBC Classic Blacksmithing Leveling Guide.