M*nards sell it.
www.dry-b-lo.com www.undercoverdecks.com www.dekdrain.com
You could also look at using vinyl & fibreglass decking that is applied to 3/4 t&g ply I have used both with success each having their own advantages. Do an internet search for those, and if you have further questions if they interest you just ask.
You can use a sealed concrete, look at our site's Greenvalley Project, I used that application for that deck.
Then there are a few composite (mostly injection molded rather than Trex style of wood/plastic mixtures) types of decking that can be used as well, these interlock and provide a water seal.
The higher end you would look at a t&g (tongue & groove) applied & sealed fir or ipe for example.