Grateful Dead
photo: Grateful Dead fan in California (more by Jay Blakesberg)

To quote the Dead's own website:

CashorTrade.org is the world's first, fair trade marketplace where fans ethically buy, sell, and trade tickets for face value. Since the organizations inception in February 2009, the community has grown to almost 49,000+ site members and another 80,000 in the social networks. Grateful Dead 50 Tickets listings on CashorTrade.org can be found [HERE].

Good luck!

Need another route? Ticketmaster also has lots of (more expensive) listings via their secondary market site TicketsNow, and then of course there's Craigslist and/or a good old fashioned miracle.