All Euro car turbo or superchargers don't come cheap compared to Japanese.
Try these sites...
www.kormanautoworks.com
www.turbochargingdynamics.com
Faster mods? Just try doing engine swap, put an M30 big six engine (85-88 535 or 635), they come easy, cheap and most reliable BMW engine. It will easily bring you 185-208 hp.
These are your best bets
Bing