Borrible links
There aren't many, sad to say, but here they are:
The Borrible Borough: http://infomatch.com/~carrolln/borrible.htm
The first only other personal Borribles page still existing on the web, this page has lots of goodies mine doesn't. A whole section is devoted to the connection between the Rumbles and a beloved British kid's show called The Wombles, and the Miscellaneous section has alternate chapter titles, foreign titles, Borribles references in other books (including one sent in by me!), and information on the Borribles screenplay.
The Borribles Online: http://www.theborribles.co.uk/
The official Borribles site. A bit sparse, but it still contains good biographical information on the author, and tells the story of how the Borribles got started due to something that happened to Michael de Larrabeiti when he was a kid.
Bookfinder: http://www.bookfinder.com/
Not actually a Borribles site, but this is my recommendation for finding the books. All three are out of print, so unless you can find them in a library, you'll have to find them secondhand. The first is easy to come by, but if you want the Don Maitz cover instead of the Walter Velez one, you'll have to look for a third edition Ace paperback. The second is also fairly easy to find, the third difficult. Good luck!