You'll be hard-pressed to find true marble subway tile--mainly because subway tile was never intended to be anything but ceramic.
When originally installed in the New York subway system in 1904, these tiles were all about pure, brutal functionality. The builders wanted a vitreous--not porous--surface that could easily be cleaned in the rare event of graffiti, dirt, or smoke. It had to be cheap. Rounded-off edges were nice, too.
Marble is almost antithetical to all of that. The most significant issue is that it's non-vitreous.
So, if you want marble "subway tile," look for dimensions. Here we have a 3" x 8" Calacatta Oro Marble tile from Bedrosian's, roughly in the same 1:2 ratio as mentioned previously.