http://www.lik-sang.com/default.php?likref=google_liksang
There is a good place to start, I ordered my GT4 from there (well the Chineese one) and the service was pretty good. Although I got worked up waiting for it, so I downloaded it and burnt it, which I must admit was a bit sad. And then to top it all off I bought it again, when the Pal version was released.
Although I think Arwin has like 4 different versions.
This "demo" of ours do have more cars (and compared to some games, more tracks aswell) then some of the games i have paid full price for. I think we are a bit spoiled from earlier GT series..