For official PIU files I recommend 2 sites, stepmania media (they have the best NXA arcade pack at moment) and PIUSM (must register to get access to the sub-forums, I'm a frequent user of this one, it's a good site)
..::PIUSM-V3::.. ..::[ Stepmania MEDIA ]::.. Tu portal de descargas para Stepmania
If you want fan songs then your choice is Pump Haven. =)