You'd probably do better finding a local farm than buying over the Web. The shipping costs will be huge. Of course, then you can only get it in season. And you might have to use whatever varietal they grow. Are you looking for specific varietal grapes, or do you just want to ferment some grape juice?
"Notorious stickler" -- The New York Times, January 4, 2004
Mary beat me to it. Many home brew supply shops sell juice for making wine. I have seen it both as regular juice and as concentrated. Unfortunately, I will assume that most of it has been heat processed at some point so that might affect the flavor and destroy some of the nuances. I am only assuming as I have not attempted wine before (though I have brewed many a batch of beer). So start by checking shops and then search the internet, I know there are a lot of resources out there.