I got my set at the Swiss dealership for Kelleners Sport:
http://www.kelleners-sport.ch/de/. I also saw them on ebay.de for a better price, but here in Switzerland you need the paperwork from the dealer/garage to get the wheels streetlegal for your car, so I needed to buy and install them from a proper dealer/garage, who will sign the papers accordingly.... the Swiss Department of Transportion is extremely strict.
Regarding your question on the 19" 255x40: The 19" will perhaps - in theory - perform a bit better than the 20" because of the lower weight of rotating mass when accelerating (under the assumption that 20" wheels + tires are heavier than 19"). However the 20" just look so much more better in my opinion. Under the following link, you will find a guy in Japan, who put 19" Kelleners on his X3:
http://www.fujicorporation.com/eng/s...26504&detail=1
It sounds stupid, but that additional 1" visually makes the world of difference between "nice" and "niiiice".
Furthermore the 19" wheels come in 8.5 and 9.5 width, so you'll have to figure out yourself which wheels will look better with the 255 tires. If you have the M-Sport package, then the plastic part of your fenders will be a tiny bit wider, so you may want to choose the wider wheels especially on the rear.
I also wanted the 19" at first because of the lower price, but I am soooo glad that I spent the extra cash for the 20". I would really suggest that you also get the 20" instead. And if you are a bit "crazy" like me, then also get the 245 and 275 combination. They will fit perfectly and give your x3 a more aggressive look. However one important thing to keep in mind is, that the 275 rear tires will increase traction, but also add some roll-resistance, so acceleration MIGHT be affected. Then again, the choice of tires, pressure and some other factors may influence the acceleration anyway.
Let us know, what you decide and post some pics of your new setup;-)