The PC version is particularly tricky because it acts as a hybrid title. It can be found with keys for the EA App/Origin, and it also has a presence on Valve's Steam platform, though its status there is also "unavailable" for direct purchase. As a result, the only way to get a legitimate copy today is through a valid, unused CD key purchased from a third-party marketplace.
Purchasing a valid key is only the first part of the challenge; getting the game to actually run is the second. Shift 2 Unleashed is infamous for its restrictive DRM, which frequently causes problems for players today.
: The marketplace of cheap, third-party keys has a dark side. Many of these keys are obtained through fraudulent means, such as buying them with stolen credit cards. If you purchase such a key and redeem it, you run the risk of having your game license permanently revoked when the fraudulent transaction is discovered, potentially leading to your account being banned. The genuine authenticity and provenance of a cheap key are almost never guaranteed.