You can't really say GNW is a good source since chances are if you are one of the top groups you will have many members, you won't be getting a pack every week also you will need to be in one of the top groups lol.

More reliable ways are Sage War, you always get one for being in top 3 on your side, Survival has them a lot in treasure room and i had few times from 12th node, buying for coupons and the recent chase event, i had one from it every weekend so far.