How can you buy a Cadillac sixteen?

You can't buy a new Cadillac Sixteen. It was a concept car, a one-off showpiece unveiled by Cadillac in 2003. It was never put into production. Therefore, there are no new ones to buy.

You *might* be able to find one for sale if a very wealthy private collector decided to part with theirs (highly unlikely, and if so, it would command an astronomical price). But essentially, acquiring a Cadillac Sixteen is not a practical possibility.