The Doonesbury Special was available on tape back in the early '80's; I know because I once rented it. I'm unaware of a currently-available version, but perhaps you can haggle a dupe out of another animation collector. Also, you might take a look at the latest edition of the Whole Toon Catalog; you never know what's gonna turn up there! And, of course, there's always eBay...
There was also a heavily-illustrated paperback detailing the making of the animated special published around the same time the show was aired.
SCOTT "Uncle Duke Is My Role Model" SHAW!
"When the going gets weird, the weird turn pro." -- Dr. Hunter S. Thompson