Here's a good resource if you designing your site yourself and want a nice background image for a certain section or your whole page.
36 Must Have Backgrounds and Patterns Resources for all Designershttp://speckyboy.com/2009/01/05/36-must-ha...-all-designers/QUOTE
Searching for that perfect background for your design can be pretty difficult, from these resources you can browse through 1000s upon 1000s of backgrounds and patterns. Hopefully this will make your search that little bit easier, mind you with this number of resources it may make it harder. Happy browseing!!!
Also, here's a page that explains how to make your own seamless backgrounds:
http://graphicssoft.about.com/library/weekly/aa080499p2.htm