Sorry for ridiculously late reply .-.
There's many DIY stores around in Sydney that I have access to however they usually sell foam for the most ridiculous prices ever or they provide pre cut, ridiculously sized pieces.
Can't buy anything online, I'm not old enough yet.
However, I've found an extremely thin mattress / cushion thing from Kmart which happened to have foam in it and chopped it open
(after I bought it of course.). It was much cheaper than the IKEA ones.
Thanks for all the help everyone!