Damien immediately recommended I use a stamp supplier in the USA who go by the fairly memorable name of RubberStampChamp.com which is obviously their web address. Even allowing for the $8 freight charge per order the stamps are around 30% of the cost for any Australian supplier, and frequent special deals make them even cheaper.
I have used them on several occasions. My pride and joy is the Shiny certified copy stamp that incorporates adjustable date wheels albeit in the US format. RubberStampChamp allow complete control over text weight, font, and layout in an easy web format so ordering is accurate and convenient.
Apart from the seal this stamp just needs the alternatives crossing out and signing. Over the hundreds of certified copies I have made over the months this stamp has saved many minutes if not hours.