Sam, I'm in Brissie, and sorry, I don't know where to buy wood. Maybe a boatbuilder? If you can use the bunnings wood to make the shape you want, maybe you can stiffen it up with a layer of fibreglass (glued with Epoxy) somewhere near the bottom. You would need to run the glass fibres in the direction of the board length. GOOD luck
Good luck wasn't meant to be in capitals, I wasn't shouting ;-)
check the plywood supliers in the yellow pages.
mr ply n wood
i think there is a boral ply too.
I have a Roarockit bag (70" x 20"). Works very nicely, but like I said, for big drops and sharp curves, it needs some help from clamps...
I've used the bunnings ply for 2 boards now, with 3 sheets it was way to flexy but 4 seemed alright, a lot comes down to length & cave. If you struggle to find supplies let me know, I may or may not have some 1/8 Baltic Birch, just sayin.