I followed the E90post DIY on this subject. Bought the parts from getbmwparts.com and the seats were USED.
Finding the seats was extremely difficult, but I was also determined. I was able to find them mislabeled as TAN COLOR on car-part.com in some junk yard in Ontario Canada. They sold them to me for 250 but I had to pay a hefty sum to get them to San Diego. When they came they were very dirty but cleaned up great with Leatherique.
I also used this website to quickly evaluate vin numbers for the correct color seat.
Hope that helps.