I want to purchase a copy of a book published by the AGPS

The Australian Government Publishing Service ceased to exist in 1997 and unless stocks were retained by the author agencies or the titles were later republished by those agencies, titles published by the AGPS or the AGPS Press are no longer in print.

You may be able to acquire out of print publications through second hand book dealers. There are many used book dealers online and a number of meta-search sites that search across numbers of used book dealers.