Retrieving old BF2 key

Unless the key was attached to your Origin account there is little that can be done. Since this sounds like a pre-Origin physical copy my only recommendation is to speak with a game advisor and see if they have any sort of access that might be beneficial in finding your key. You can contact a game advisor here:

1 Click on the following link: https://help.ea.com/contact-us
2 Make sure you are flogged in (top right corner)
3 Click on the blue button "Contact a Game Advisor"
4 Enter the name of your game or product (or "Origin" for account-related issues) and click "Find Solutions"
5 Select the type of issue and the platform you play on, and click on "Find Solutions".
6 Click on the blue button "I still need help" if the FAQ articles do not apply to your situation.
7 Select a contact option. Note that you may get different contact options depending on your country.

I recommend that you use the telephone option and the “call me” option so you don't have to stay on hold for a long time (call back feature available to North America only).