All 11 issues are available from the "Feature Articles and Interviews" section on the right side of the page, and from the official Robert Shea Web site, which you should look at, as it provides free ebooks of his solo novels and other good things. As you might guess from Shea's description of his zine, much of the material will be of interest to Illuminatus! fans.
There may only be 11 issues of the zine, which was published irregularly in a span of about 15 years; issue no. 1 is dated "Spring 1975," while issue 11 is dated "July 1990." The Labadie collection at the University of Michigan, which has kindly made these issues available to share with you (after I got permission from Mike Shea, Mr. Shea's son), lists only 11 issues.
I have made inquiries on whether there were any other issues, but apparently 11 may be it. Robert Shea died on March 10, 1994 but perhaps he turned his full attention to novel writing in his later years.
The Shea Wikipedia article, by the way, states that "The zine was mentioned in and read by one of the characters in Illuminatus!" Can that really be true? I could not find the zine when I ran a search of the text in Amazon, and the work was written in 1969-1971. Can anyone help?