Since the game is delisted, the only way to get a digital Angry Birds Space activation key for PC is through gray-market key resellers. These are sites where users bought keys years ago and never redeemed them.
Even with a valid key, the game often fails to activate because it tries to "phone home" to a server that no longer exists . To get around this, players typically use one of these methods:
100% legitimate. No malware. You own the physical media. Cons: You need a DVD drive; the key might have been used already (though rare for PC discs, as most used SecuROM—check the listing).