Adding more to this list:

1) sub-plot mission of reaching a certain levels i.e. level to 50, 55, 60, etc.

2) Weekend missions - puzzles, lucky throw, TI scrolls (I think ?)

3) Myoboku Trial and other random events throughout the month that gift coupons

4) Wishing tree - max growth the level of the tree to be given options of rewards (one of them is coupons)

5) Ninja Bonds - You can earn coupons from the ninja bond when it reaches level 5, 10, 20 etc

6) Ninja quiz - get 10/10 correct and get a chance to get coupons

How long have you been playing? If you've been playing for a few months, I can see why you don't know where to get certain items. I've been playing since December.