If you didn't already know about it, I stumbled across the internet archive for Grateful Dead live tapes. You need to search and apply filters, but there are both audience and soundboard recordings of shows from 1966 to 1995 available either to stream or download. If your Dead obsession is also high end, you can download about 3000 or so in FLAC, although the fact they were usually recorded on a single mic in the taping area by a stoner does suggest that 320kbps mp3 is probably as worthwhile.
Currently enjoying a 30 minute version of 'Dark Star' segued into 10 minutes of 'Morning Dew' from Winterland in 1974 whilst downloading the 3hr plus set!
Totally legal - it is an online repository for important cultural artefacts. Whether Jerry and the band playing 30 minute versions of 'The Only One' and 'Weather Report Suite' counts as important I'm unsure, but hey!
Oh, and steer clear of the 1969 Fillmore East show, Dark Star is just getting brilliant when this annoying girl starts asking people for a joint for the next ten minutes at high volume with 'Valley Girl' accent.