A Betty Board (board is short for soundboard) is a bootleg copy of a series of Grateful Dead shows known for the incredibly high sound quality. The story behind these shows (62 of which are in semi-wide circulation) is quite remarkable.
Betty Cantor was an avid fan of the Grateful Dead and taped many of their shows. For whatever reason she abandoned her collection in a storage facility in 1986. Eventually the contents of her storage facility were sold in auction due to Betty not paying her rent. The tapes were sold to several people, and one such batch of tapes was re-circulated in 1987
The Betty Boards revolutionized tape trading as they were some of the highest quality recordings to survive the late 1970s and the 1980s.
What made the tapes so damn good was that they were products of two track reels spun directly from the soundboard with no salting to degrade the sound quality. As a result, the high quality is immediately noticeable. One can hear the drumstick hitting the metal disk of the cymbal as well as the resulting shake and wobble. Furthermore, although not directly related to sound quality, one gets the distinct flavor that reel-to-reel taping has from these shows, something not found in either later soundboards, or the soundboards released by the band.
I would suggest www.etree.org for any and all of your tape trading needs.