Mark,
Although they are softwood plys, Hoop Ply and Marine Ply both do an excellent job, what do you intend to do, build a telescope or drive the car on it ? I know Andrew Murrell's 20" Hector is built from Hoop Ply and he is 15 years old and still going strong. When Gary Kopff set up the Argo Navis Pointing Analysis software on it a few months back, the summed pointing errors were less than 20', which aint bad for a 15 year old telescope built from softwood plywood
The main thing is to stay away from home improvement stores/centres and go to a specialist supplier of sheet timbers. Any high grade plywood suitable for exterior use will be good. As Ed says we have Mr Ply & Wood and Plywood Express in NSW which are both specialist suppliers. Any store like this will have a material thats suitable.
CS-John B