You'd be amazed what you can do with pure CSS these days, however there's loads of sites out there offering stuff for free. Here are a few of my favourites..
http://ui-cloud.com/ (Search engine for UI graphics)
http://subtlepatterns.com/ (Patterns for backgrounds)
http://www.dinpattern.com/ (Patterns for backgrounds)
http://365psd.com/
http://www.premiumpixels.com/
http://www.designkindle.com/
http://designmoo.com/everyone/
You can also search on Dribbble.com for terms like 'free' or 'psd' as there's a lot of talented, generous people on there!
Be sure to read the licence of anything you download from these sites, often the author requires attribution or doesn't allow their use on commercial products.