ShadowQuilava

In the top room on the left wing of shade's gym

In the locked room at the back of Shade's gym. You need a special Yureyu Key to unlock it, and that key is found at the end of the Wasteland's strength puzzle. Better get it fast, because Magnezone will be postponed even further into the game in ep16 onwards.

Theorically it shouldn't be a problem to get it if you still keep your copy of E15 tho, but yeah it'll probably be moved