Wolfgang's Vault has made some classic Boston shows available for purchase online. The shows can be purchased as mp3 or FLAC (Free Lossless Audio Codec). There are currently three shows available, all from the King Buscuit Flower Hour recordings:

Live @ Spectrum - Philadelphia, CA (Dec 18, 1976)
Live @ Long Beach Arena - Long Beach, CA (Mar 16, 1977) - Early Show
Live @ Long Beach Arena - Long Beach, CA (Mar 16, 1977) - Late Show

The FLAC versions cost more, but will give you better quality than mp3. If you don't have the ability to play FLAC files, you can get the CODEC at flac.sourceforge.net