Finding issues of Assembling magazine (or copies of any magazine printed and assembled by its authors) can be difficult. You can't buy copies on ebay or Amazon. Press runs were limited to the number of copies submitted by the writers, and distribution seems to have been largely a matter of writers mailing copies to their mothers. The most widely available issue of Assembling is the 9th, which, because it was produced with an NEA grant, had an actual press run, and although the focus is on criticism, it includes many fascinating creative pieces. Another widely available source is Kostelantz's 1975 book Essaying Essays, a collection of alternative, multimodal essays that includes several pieces reprinted from early issues of Assembling.
Unfortunately, little of the work from Assembling is available online. It was all published long before personal computers and the Internet, and so far, no one seems interested in archiving this early history of multimodal composition. Indeed, I am not sure we even know how to digitally archive this sort of ephemeral, pre-digital work. Yale's digital image database contains four images from Assembling, and while Kostelantz's own web page has little information about the magazine, two of his collaborators, Mike Metz and Karl Young, have personal websites with some archival materials.