If you can find a source of foamcore in Australia at non-ridiculous prices, let us know. As far as I can tell it would be cheaper to buy inserts.
A quick google search found Draftex in Sydney:
Based in West Ryde and appears to have pickup available.
A3 5mm White Box of 10 = $22
A3 5mm Black Box of 10 = $26
Never used them myself, no responsibility taken, YMMV, buyer beware etc etc.