Check RealOEM to see what they list as the space-saver kit for the X3/X5 and see if the parts are the same as that of the E90 etc. There are loads of different ones, but the X5 space-saver is quite massive compared to the E90 since it has to have the same tyre diameter etc.
I have seen a few adverts for loose 17" space-saver wheels for the E90 - seriously, take if you can find those! The bag and jack can be bought later and can be something else for that matter, it's the wheel that is critical of course.