Bunnings sells 5mm sheets of white foamboard for around $7.50.They also have black,but for some reason that is priced about $9.It's 30x20".
The white stuff has a semi-gloss paper on both sides,which takes paint very well.
Since using it in building my lazy bee,I've learned that you're supposed to soak at least one side of paper off,where the foam is going to be glued.
I used mostly hot glue,but the premium Gorilla glue is highly recommended by most foamy builders.Also available @ mr.Bunnings emporium.
Pity they don't sell any of the 3mm stuff.The price of balsa has really skyrocketed in the last few years
Oh,before i forget,they had a 2000w heatgun with 2 heat settings on special for $20!Bargain!I'm tempted to go back and get a spare one just to chuck in the cupboard.Heat guns don't last very long if used to strip paint off the house,which i believe was their original design function.