You can still find digital Steam keys for on various third-party marketplaces. Be aware that because the game is delisted, the supply of these keys is finite, which can sometimes lead to high prices.
If you are looking for a way to get the game today, here are the available options: dirt-3-game-link
If you find an unsealed physical PC copy, the "Live Access Code" found in the manual can often still be used to activate a product on Steam. This usually grants you the , which removes the old "Games for Windows Live" requirement and adds all DLC. You can still find digital Steam keys for
: A primary source for used physical discs and older "Games for Windows Live" PC boxes. 3. Activating Old Keys This usually grants you the , which removes
: Lists various Steam keys and gifts for different regions. Instant Gaming : Often carries keys for the Steam version. MMOGA : Provides Steam keys for the Complete Edition . 2. Physical Retail Copies