Where can you get a legal replacement key for Doom 3

You'll find a CD key in the box when you purchase the game. You can find it at a number of video game stores and their online counterparts.

If you're looking for a physical store, you could always try your local GameStop. You'll also probably be able to find a copy on the following websites online; Amazon, Ebay, Best Buy and the GameStop website, to name a few.

If you already purchased the game, but lost the code, you can generally get a new one by contacting the games publisher.