[Chao Freak]

I think you can get custom songs here.
åˆ�音ミク -Project DIVA- 　エディットデータアップãƒ*ーダ
I still don't have the game so I can't check but... Good Luck!

EDIT: More here.
åˆ�音ミク -Project DIVA-　EDITデータ専用アップãƒ*ーダ
Just so you guys know, they don't have the mp3s so you'll have to get them yourself, although most of the files have a readme with a link or a nicovideo number so you should be alright.