Usually only the most recent 1-4 issues are available as backissues. $5 each includes shipping. The best attempt is made to print enough to supply all immediate demands so there are a number of copies available for months after the issues print date. It is difficult to guage. Backissues are listed as available on the main page and removed as they sell out.
Older issues no longer availabe as backissues can be requested but can only be photocopied in B/W. (However, advertised reprints have color covers and plates like originals) Unfortunately copying an old issue costs more money and takes much more time than full print runs so photocopies are $7 each including shipping.