Try here:
http://www.birds.uk.com
Description : Sport Suspension Springs
Manufacturer : BMW
Vehicle : 3 Series (E90)
Part Type : Suspension
Application : 320d, 325i and 330i, 30 mm lower
Installation cost : £218.00
Parts cost : £232.14
Possibly not cheapest about but they are a BMW dealer and approved Hartge dealer/installer.