Where do you get the key to unlock that door in galactic ware house veilstone city Pokemon platinum?

You can't unlock the door in the Galactic Warehouse in Veilstone City with a key in Pokemon Platinum.

Here's how you actually access the area:

* You need to defeat Team Galactic's leader, Mars, in the Veilstone City Gym. This is a crucial step, as the warehouse door opens *after* you become the Veilstone City Gym Leader.

* After defeating Mars, you'll have to travel to the Veilstone City Department Store. Inside the department store, you'll be able to access the back room through a door on the second floor.

* Inside the back room, you'll find a hidden passage that leads directly to the Galactic Warehouse.

There is no key involved in opening the door to the Galactic Warehouse, but it's a significant plot point that opens up new areas and challenges in the game.