The online version of the "Guide" is hosted by the BBC and is called H2G2. Here's one entry about the original "Last Chance To See" radio series, as broadcast on Radio 4.
As I'd discussed with MJ Simpson, it seems that the series is somewhat hard to find, because its never been released commercially. Hopefully, with all this new interest in Hitchhikers, that will change. I do recall listening to it myself, and I know I taped some episodes at the time, but they're long gone. Anyone out there with a copy?