Not sure fire by any means, but Strike playlists, public events, and rare engrams all give you chances to get a strange coin or two. They'll also drop as post match rewards in Crucible. I got three last night (one from a gift package from New Monarchy after hitting rank 3). You shouldn't be too hard racking up 5 coins by Friday doing this. Come tomorrow, after the weekly reset, you can earn coins from the Weekly Heroic strike too; 3 from each difficulty level to give you a total of 9 for the hard difficulty.