Hi Rob,
3/8 Hard drawn Copper tube should be available from a plumbing shop ( maybe to order ??) Try Reece?
I found some offcuts in the ceiling of the local shopping centre from the refrigeration mob installing the fridges into woolies, so you must be able to buy it somewhere.
The Copper sheet for discs may be available from George Weston in Brisbane ?
The annealed coil copper can be straightened through a tube straightener but that?s another tool to buy.
Keep us posted on the radiator build
Cheers Justin