Want to play Supreme Commander: Forged Alliance

My cousin recommended me to play Supreme Commander: Forged Alliance because she knows that I like real-time strategy computer games. For that reason that I downloaded it in Amazon and installed it in my laptop. However, when I opened it, a message prompted telling “Wrong disk inserted. Please insert the original Supreme Commander: Forged Alliance CD/DVD.” That’s when I realized that I downloaded an original disk and now I can’t play the game. Could you please tell me how I should go about this? Thank you very much in advance for your help.
