I believe you need to find someone who owns the game and then ask them for one.
If you yourself own it and are asking how to get one for a friend then you need to check whatever email you had the game registered with as I believe they sent it to you that way.
I don't know however if you can keep the account you used with the buddy CD-Key after the 7 days runs out. If you can't then it's pretty lame imho.
Good luck with it.