something like the H&R touring cup kit which includes both struts and springs.
I believe koni and eibach make similiar products.
Will also depend if you are on OEM regular OR OEM sport suspension, and if you have a AWD or RWD car. Those factors + how much you really want to drop your car will tell you which setup to go with.
I am also looking into lowering my sport-suspension e90 down... leaning towards the above H&R kit.
2008 328i E90 Jet Black:6MT:Premium:Sport:Walnut/Black interior:Xenon:MTEC V3 AEs:19" Beyern Mesh:35% Tint:Blacklines:OEM Front Splitters : OEM style CF truck spoiler : CF mirror cover overlays