From a UK perspective there is lots of choice for 4" Speakers. They can easily be made to fit with adapters. However for the 8" Underseats there is basically no choice. Earthquakes are very expensive and Jehurts and MB Quart's can only be bought as a complete set. The subs / mid basses can't be bought separately. Thus in my opinion you would have a far great market if you concentrated on this option.
E90 330i SE Auto Graphite Grey Metalic