Working key stolen, Help please

[LoudAzzCat]

I bought my 03 X5 about alittle over a month ago with one working key and one that starts the car but wont unlock doors. Last night someone stole the wifes hoodie with keys in pocket. Unfortunately that was the only key remote that worked. Ive drove many times with the nonworking key in hope that it would recharge with no luck. I was wondering if I should try to change the battery in the one remaining key or will that affect the ignition part of the key. Can I find the battery at a local auto parts store? What should I do? Ive read about nav ignition imobilization. Im kinda lost right now and dont want to make numerous trips to the dealer at over 1 1/2 hour drive one way. Helpful comments appreciated. Thanks