The books US publishers, Schifferbooks in Pennsylvania seem to have an adequate supply.
http://www.schifferbooks.com/newschiffer/book_template.php?isbn=0764323008As a hypothetical, I successfully added 1000 copies of book 3 to my shopping cart for a grand total of $59,950US.
Their website lists Peribo Pty Ltd as their Australian distributor.
Peribo Pty Limited
58 Beaumont Rd.
Mount Kuring-Gai, Australia, NSW 2080
Contact Name: Michael Coffey
Perhaps Mr Coffey should be contacting the US publisher for a resupply of stock since they "appear" (based on statements provided in this thread) to be having supply chain management problems (and are thus losing sales to their US competitors)?