So the main sources are either EA's store (Origin/EA Play) or third-party retailers. If EA still sells the game, they can buy it there. If not, then third-party options are the way, but with a warning.
Also, legal notice: Emphasize the importance of legal purchases and not sharing CD keys. Harry Potter And The Half Blood Prince Cd Key
Also, some people might be looking for a free CD key, which is unethical and often a sign of a scam. It's better to advise legal purchases. So the main sources are either EA's store