One of the developers tweeted that you could earn them in game, but so far none of the reviewers (or people who got the game early) are reporting that they've got any in game.
Could it be possible that they will activate/patch them in once it releases so that people who got the game early don't get a head start on acquiring those coins?
Retail games shouldn't have microtransactions at all but when you have more expensive microtransactions packs in sale than the base game there is seriously something wrong. Hopefully they haven't made acquiring them in game a pain in the ass.