You can either obtain your own dedicated common short code or use a shared common short code available from a mobile marketing company.
Dedicated vs. Shared: Go to this page to read a comparison of dedicated short codes and shared short codes.
Mobile Aggregators: Go to this page to see list of mobile aggregators that can provision a dedicated short code for you.
Mobile Marketeers: Go to this page to see list of mobile marketing firms that let you leverage an existing shared short code in the US.