I bought my dampers and springs a while ago, so I am not sure those places still have the best prices.
If you check the Sponsors section, you can find a lot of vendors carrying H&R, pretty sure you can work a sweet deal:
http://www.e90post.com/forums/forumdisplay.php?f=203
Bilstein Sport dampers - Turner Motorsport or Bavarian Autosport carry them, just make sure you get the xi parts numbers.
If you still have troubles finding these vendors online, PM me and I will send you the links - as I don't think they are forum sponsors.