MTBR.com will help you find some sources!
Here is a list I have on had and bought from before:
http://www.cambriabike.com/
http://www.beyondbikes.com/
http://www.chainreactioncycles.com
http://www.nashbar.com/index.cfm
http://www.performancebike.com/index.cfm
http://www.pricepoint.com/
http://webcyclery.com/home.php
All of those shops seems fine except Performance Bike which is a huge US store, great product, great price, but not customer friendly if you ever talk to them on the phone or need support.
There are also store which sells specific items:
(i.e.)
http://www.aspirevelotech.com - Chris King
DID I MENTION EBAY? I HAVE BOUGHT 170+ BIKE RELATED ITEMS THROUGH EBAY FOR THE PASS FOUR YEARS!