Buy them when they go on sale. It's sometimes been right after the cruise and sometimes it's a couple months after. If you haven't already, introduce yourself and get to know some people on here and often times people will select others to be their cabin mates in advance and one person will be responsible for booking and get it all taken care of.
MDF: 2014, 2015, 2016, 2017
Summer Breeze: 2014, 2015
ProgPower: 2014, 2015, 2016, 2017, 2018