There are lots here on this site. You can also make your own using any image you so desire and having the CrackBerry Wallpaper Maker resize it for you.
To change the wallpaper, go into your Media folder, go to Pictures, select the one you want, and click Set as Wallpaper.
Posted from my CrackBerry at wapforums.crackberry.com
I've been lovingwallbase. Sometimes I'll pick them using the phone browser, but it works better to just look at them on a pc and then email them to yourself.