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.
Happy hunting!
SCOTT "Uncle Duke Is My Role Model" SHAW!
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"When the going gets weird, the weird turn pro." -- Dr. Hunter S. Thompson