Alooha CWayte! The best place to get cosplay foam in Australia would be from Lumin's Workshop. It's EVA foam, the same type as gym floor mats are made out of, but this cosplay foam is denser than the floor mats so it's more rigid and sands better (not to say people haven't pulled off some amazing armour with floor mats, which many have). The downside is it does cost more, so it's personal preference which you go for.
You can get quite a good finish with foam - not mirror quality, but great for most armour. That also depends on how you seal it, such as with Mod Podge, Flex Bond, or Plasti Dip - though this last one is quite expensive over here.
The two other major types of materials to use to make armour would be 3D printed, or using the pepakura (paper craft) method.