I hadn’t realized until flipping through Previews the other day that Steve Jackson Games still has Principia Discordia in print (and at a very reasonable $11.95). The book, of course, has also been effectively in the public domain since its 1965 publication and can be read online in a number of places, including PrincipiaDiscordia.Com.
The SJ Games version does includes “20 pages of new material, as revealed by Eris to her faithful worshipers when they probably should have been mowing the lawn or something.”