Dude,
I'm after the same thing (rear window spolier) i think they look well smart... the slimline ones.
Anyway, as for the shark fin, forget it. The BMW fin has a flat base, but our OEM aerials are on the curve, so it will never be a perfect fit.
I have however found a fin that does fit smoothly, and has stock connections (not that i'm using them) but it looks smart. PM me for details.
Let me know if you find a rear window spoiler.
Adios amigo.
B)