If you are referring only to files on the Gnutella (G1 network) then either GTK-Gnutella or WireShare would be best answer. I cannot say about other networks. The reason I suggest these programs is their superior search ability for the gnutella G1 network including DHT and their ease to connect to the network compared to other older programs. Both ares and shareaza use primitive gnutella G1 search technology (almost 15 years out of date.)

People often only share files for a period of time so perhaps they are no longer sharing those particular files. Also unfortunately some people only share particular files for the session they downloaded the same files.