Here's three excellent sites for finding free textures. All three allow use for personal and commercial use:

http://www.cgtextures.com/
http://mayang.com/textures/
http://textures.forrest.cz/

Textures are great to use as backgrounds for your headers and banners.

Paste them into GIMP, then use the techniques in the videos above to add text and you have access to unlimited headers for all your sites.