Motor Forges are the shops of Brütal Legend. Here you can purchase upgrades for your car, axe, guitar, and other things.Before being unearthed with the Relic Raiser solo, they look like giant engines sticking up from the ground. After raising them, drive the Deuce through the door to visit with a very recognizable metal master. Speak with the Guardian of Metal to purchase upgrades with your earned fire tributes.
There are 11 of these scattered throughout the world, and though they're not too terribly difficult to find, we've provided a link to a useful map if you require help.