HHGG Radio Plays
Here is a link to Amazon UK where you can purchase all 5 of the radio plays in one box set. Even with package and posting it's still cheaper to order from Amazon UK than other online houses in the US: about $74USD (41GBP)with S&H vs. $90USD w/o S&H.
http://www.amazon.co.uk/exec/obidos/ASIN/0563504196/qid%3D1138241089/026-6982704-3226827