Hey there, I'm Brisbane based but I've heard that Riot Art supplies sell "Closed cell PVC foamboard" which is the same as sintra. Or try contacting signwriters and graphic companies. They call it Foamex but it's the same thing. I've also heard stories where people have gone to graphic's companies and been able to get offcuts of Foamex for free so it's worth calling up different companies and seeing if they can offer that. Everyone loves a bit of recycling.