Hm, aren't there millions of "tape trading" circles on the Internet? Must be plenty of band specific newsgroups (bootleg newsgroups and JA, Clash) to post in. I've seen band specific forums ( JA, Clash?) with bootleg trading sections.

Usenet may even have bootlegs in their binary sections if you're patient enough to wait for it. You could even try Bit Torrent for concerts, but I don't know links to any non-spyware'd search engines...... but I can't imagine these downloads are legal.