The Pre, out of the box, is pretty robust and complete. It has a nice selection of wallpapers and some OK ringtones. But, with anything, there is room for improvement. I am compiling a list of sites that contain new wallpapers and ringtones. The list is by no means a comprehensive list, but should get you going without having to look too far.
Everything Pre – Wallpapers There are quite a few wallpapers here
idesignpre – lots of interesting wallpapers, including some pretty people
Pre-Wallpapers.com – LOTS of wallpapers, some of which are NSFW
Of course, both the wallpapers and ringtones are easy enough for you to create on your own. Wallpapers need to be 320 pixels wide by 480 pixels tall, 24bit color or monochrome. Ringtones need to be MP3 files. You can use any MP3 creation tool to create a short MP3 track. You should keep the tracks to under 20 seconds. You can use a complete song, but that would not be a good thing to do. Most ringtones are under 20 seconds. For wallpapers, pretty much any image manipulation tool that can save to JPEG format can be used, included Paint in Vista and Windows 7.