Contact Subway corporate to see if they have any available space in your city. Last year when we checked with them for Houston, we were told that they do not have any opening / location in Houston surrounding for new Subway. This is from my own experience. This was for standalone subway. It might be different if you are planning to open subway in a mall or within other business complex.

Of course you can buy a running subway, but remember all subways are running in profit...be prepared really high price for it. last year, in state of texas, there were 2 subways for sale (in subway website), both in the border of mexico (guess why?).