There are only a few brands who make OE keys. I would start with a BMW dealer's part center which should have a kit which includes all of the OE key options. If you find a match, you can order from McGuard or whoever makes it using the key's ID#. If you can't ID the key you need, there are other ways to remove it, but there is a risk of potential damage associated with some of the methods.
Let us know if you find it! Good luck!
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