All the usual suspects have it (Home Depot, Rona, etc).
I've been looking into it to but it's pretty expensive, and it's not watertight like the vinyl decking surface is (which if it's a high deck and you want to use the space under it in the rain is important).
There's some aluminum stuff that I've been looking into which seems pretty cool, it doesn't allow water through:
http://www.wahoodecks.com/