It's Fu.ck the Pain Away (minus the dot) by Peaches and its on the Teaches of Peaches album.
they have it on KaZaA and WinMX and probably most of the p2p networks.
You could also try soulseek.
you can probably get the whole sound track on KaZaA, i have seen Kaze Wo Atsumete on there and City Girl and Too Young (which is on the Pheonix album called United).
but i think after you have sampled some of the songs by downloading them you sould think about buying the CD, after all Sofia Coppola and many others have put alot of time, effort and money into the film/music and deserve to be paid for it.
the cheapest i could find the soundtrack for was £8.36 (with p&p, before p&p it was £7.12) through amazon market place and that was sold by www.disctrade.co.uk
, and the delivery was quite good since it came in three days (this was delivery to the UK btw).
If your in america im sure you will be able to get it for cheaper. You could try the amazon marketplace at amazon.com