SOG doesn't handle the MNSSHP or MVMCP tickets. You can get them at the park gates or at the various guest services outlets in WDW, including 2 places in DTD. We bought our tickets at guest services in DTD when we first arrived for our last trip, about a week before we were going to the MNSSHP party. The DTD outlet in the Marketplace area was open 'til at least 11pm, maybe 11:30. So that's convenient hours. It's rare for the military ticket availability to sell out, since the dates are usually just in September. In 2012, on Oct 1st, WDW added 4 dates in October for military discount. But you can't count on that to happen again.
2012 ticket prices were announced in this forum on May 1st, 2012. Hope we will be hearing soon, with dates and prices for 2013.