P2P certainly is the way to go, what you are stuck at is the 'Discovery' bit of the process. I had co-authored a book, just on the subject when P2P as notion was pretty much still in its infancy. I tried re-creating Nabster, minus the lapses that ran it into litigation problems, and gave away the source code free with the book. The book is still hanging on by its fingernails, has survived heavy pounding from critics, and has already been published in few other languages. Can be found here,
http://www.amazon.com/gp/product/076...pr_product_top
Obviously, I love this book ;)
Regards
Ankur