Strange coins can be received at random from decrypting engrams or other rewards for daily events but it is rare. But they are the guaranteed reward for doing the Weekly Heroic strike. (not the nightfall), The strike will have 3 levels of difficulty and you get 3 coins at the lowest level, 6 for the medium level and 9 for the hardest level (level 30). This strike uses matchmaking now so its easy to do this with 2 randoms or your own fireteam of 3.