PSP plays music in MP3 format, so you’ll need, which you can transfer MP3 music. You can free PSP Music, with the peer-to-peer programs like Kazaa, LimeWire, etc. With these programs you can download for free the music of other PSP users in the world. But despite the fact that this music is free, this method has a big disadvantage. The files are often of poor quality. I also downloaded a large number of songs with viruses. So, if you like, in danger - you can try these programs.
There are many pay-per-download sites, where you can create a high-quality music in MP3 format, but if you want to download hundreds of songs, it will be very expensive. Also, I do not recommend that you use these sites.