The jersey your looking for does not exist for sale. It's a great mystery to me, because they are by far the best alternative jersey in my opinion, but they have never been for sale or been worn again by the team since the season they broke them out a few years ago.

That's because they are not alternate jerseys, they were retro jerseys worn - as mentioned by lon3lytoaster - during NBA history week back in December 2007.
The only alternate jersey the Celtics wear regularly is the green/black one already posted. The Celtics also wear a St. Patrick's Day jersey in March for that week's games (the Bulls and Knicks also have jerseys for that week). The alternate jersey, along with the regular home/away jerseys are the only ones that we should see for sale this season.
I would love for the St. Patty's Day jersey or the Russell Era to be available, but I doubt it.
Yeah, a thread about this specific jersey pops up every few months so it's not the first time we've had this conversation.
And I distinctly remember them wearing that jersey a few different times during the season, not just during that one week, but I get the gist of what your saying. Either way, they only wore them that season, and they've never looked back.
I wish they would though.