While hunting the internets for Hipopo fanart (I know it exists ._.) I came across this.
http://popn.skr.jp/top.html
This is a Pop'n music fan search engine. Wanna find some art for say... Hayato? Sure thing! Just type in Hayatos name (Katakana usually, but i got results for english names too) and you'll find every page that has tagged Hayato.
Want to get some pictures related to say... Pop'n AC 14? Typing AC14 (or like CS8) will get you everyone who has that tag. It is amazingly useful tool for fans to find fanart for their favorite character. (Field day for Roki :3) Furthermore for that Des-group or the Devuil there are preset group search too. Not sure bout those school group thing (ala Hayayo/Taro/Najaki/Sora/Etcetcetc) and lots of union sites exist too, featuring links to all fans related to that union (from the basic Hipopo Tamayo union to the Dave/Roki couple union)
Anyway. Old or not enjoy!