I had a set of Schnitzer Type-IIs (silver, one-piece) in 18x8.5 on all four corners of an E39 540i six-speed (before the OEM set-up was staggered). I bought them from Discount Tire in Jacksonville, FL (same parent company as Discount Tire Direct--see Car & Driver ads) for $520/each, which was well below any other place I'd found. (At the time, I installed BFG g-Force KD's at 245/45-18 for about $320 a corner).
While the Type-IIIs will obviously cost more, they'll probably be less than what you've found. I'd try calling Discount Tire to see if they can order them for you.
I have the AC Schnitzer type III wheels on my carbon black 2000 M5. They are chrome and are a little on the expensive side. I kept the same size wheel as stock. (18s) The total package was about $5k. Very easy to clean and when they do get dirty, they look like the OEM wheels clean!