If you're level 27-28, run the Weekly Nightfall and Weekly Heroic strikes every week. The weekly heroic is guaranteed 9 coins if you complete it on 28 and the Nightfall has the potential to reward you with 9 coins as well (there's a variety of rewards you can get but there's still the chance of getting coins). Other than that there's engram farming and public events.

If you're a high enough level the weekly strikes give you up to 9. I get a ton from engrams however. I think I have 34?

I almost always get them in strikes and crucible. Try the weekly and daily.