If your paper maps are not too old, scanning them might be the best option as I don't know any source for the rooftop maps.
Heme no problem though...
In case you do want to buy them there plenty of options available, like
[Please Register or Login to download file] .
Obviously it depends on the region you are after as it might be hard to find a Otways map if you check a shop in Darwin
Most shops I know only stock some local maps and considering the waste amount of rooftop maps .....