Easy:
1. Move to Europe, preferrably Germany if cost of the car is an object (try to avoid countries with high taxes on luxury cars like Norway or especially Denmark)
2. Get used to the concept of cars in general (and also gas) costing somewhat more than what you were used to so far
3. Walk into your nearest friendly BMW dealership and simply order the B5 of your dreams (check the configurator on the Alpina website). Should you consider a test drive, look up Alpina dealers or country distributors on the Alpina website as they usually should have demo cars on their lot. If bought in Germany, factory delivery in Buchloe is optionally available for +600€
4. Wait about 4 months (a "Lavalina" customized leather interior takes a little longer) and register the car locally
5. Enjoy a very refined and exclusive kind of BMW and have a big grin on your face every time you're sitting in it