You can use the Amazon deal to get mp3's, Or you can do what most Sidekick users have been dying to do: Use your own mp3's as your ringtone. Just add some mp3's to the included SD card and voila, Menu>Set as ringtone.
There are a TON of Wallpapers of awesome quality (including an Android pissing on an Apple, Which is my favorite and what I use) at
www.g1wallz.net or .com either way have fun.