Hello and welcome
I will be honest which I trust will be appreciated.
You pretty much have only one reasonable option at this time, and that is to purchase a new key from a BMW main dealer. This is the only place where you can buy a key that can start your car. You can but "blanks" online, but these are nothing but remotes, they cannot be made to start the car through any means. BMW will require proof of ownership and identity before they order you a new vin-specific key
the battery in these keys is not meant to be replaced, and it is not uncommon to damage the key when the battery is attempted to be replaced.
these keys have chips in them that are read by the immobilzer on the car. Without the chip, the immobilizer will not enable start. The only work arounds besides getting a new key from BMW would be to ship out your ECU for grey market reprogramming (would cost more than a key) or to replace the key, key cylinder, immobilizer and ECU of your car with those from the same donor car (again, much more costly than a new key)
Sorry if you were hoping for a different answer, please don't shoot the messenger - I can only give the honest answer, which I have. Please remember that a low rating PUNISHES me for my honest and prompt response, and I receive nothing for my time and answer until a positive rating is clicked.
Personally, I would recommend that you purchase two replacement keys. all it takes is for one to be damaged or lost for you to end up stranded.
Please let me know if you have further questions. Please do not forget to click a positive rating as this is the only way I receive credit for my answer, thank you. Best wishes with your vehicle.