Looking for a futon sofa mattress in Sydney

Looking for a new futon sofa mattress in Sydney, but there doesn't seem to be a good range.
It's for a typical metal futon sofa (think cheapy Fantastic Furniture style). The old mattress was a foam crumble type, so I'm happy to get that again, but the cheapest I've seen was $175, which seems a bit expensive for what is essentially just a futon cover stuffed with crumbled up foam bits….. anyone know of a good place to check out these mattresses? Every shop I've tried just sells the complete sofa and don't know how to get the mattress alone (grr!)