I do a copy paste of my answer.
I used 3 websites on my search for new rims.
http://www.bmwstylewheels.com/ All BMW style wheels.
http://www.willtheyfit.com/ Euh, will the new wheels fit, you get a answer to that questions.
http://www.leebmann24.de/ Shop with OEM parts, you get a idea for what it out there, ET values and stuff to compare with the X1 stuff.
Note, you could use spacers to up the ET value (or just bring out the rim to the out side).
xdrive loves same setup on front and rear, I ordered 2 time the front rims set.
I'm running the M performance 20" (8J with 225 tires) from the 3 series.
The 6mm difference (on ET value) isn't a problem.
(I have 20mm spacers at the rear just for optical purposes.)
21" 8,5J from the 7 series wouldn't work.To bad because they have some bad-ass bi-colors.