I am having the same issue with the same air spring. In fact I ordered and just received the air spring today. I plan on installing next week. This is where I bought the spring.
getBMWparts.com powered by Tischer BMW - Your #1 Source for all OEM BMW Parts/Accessories!
Tischer has it for just under $160 and my local dealer sells for over $300. There is a recent thread with a DIY on the air spring.
http://www.Xoutpost.com/bmw-sav-foru...pring-diy.html
I feel you on the nothing but issues with your X. I have had mine just shy of year and have had to do the following major repairs.
Transmission rebuild
valve cover gaskets
2 ignition coils
Radiator and hoses
Water pump
driver side top strut mount (I think the bad air spring had something to do with this as the mount was replaced when I lowered the X.)
Welcome and GL.