So at least for this month, we still have seasons one through eight of Dexter on Netflix. I would warn you though, if you have never watched either season, make sure you give yourself about two days (with breaks to eat and occasionally sleep)! I didn't start watching Dexter until we moved abroad a few years ago and spent from Friday after work until Monday am watching the show.
Are you currently located in the U.S.? If not, you will need to look into a U.S.-based VPN service that will allow you to use Netflix while not physically located in the States. If you try to login while abroad, standby for the "Not offered in this location" error! Enjoy watching Dexter either way.