The NSW and SA Heron Associations maintain listings of boats for sale. Interstate boats are also often listed on these sites. Listings can be accessed by the following links:
NSW Boat Listing SA Boat Listing
Herons for sale can often be found by searching in Gumtree for Heron in Boats or Boats-from-AU.com under Heron
Here is a Heron buying guide that discusses the variations in Herons that come up for sale.