Items, NPCs, quests, objects and spells of patch 1.12.1, read live from the realm itself.

Practice Lock

NPC	Level	Zone	Chance
Horde Peon	26 - 27	Ashenvale	0.06%
Rotting Slime	20 - 22	Ashenvale	0.052%
Ashenvale Outrunner	23 - 24	Ashenvale	0.02%

Object Zone Chance

Rusty Chest 4.80%
Source How Reagents

Practice Lock Engineering (1) Bronze BarHandful of Copper Bolts ×2Weak Flux