Well, I can list a few ways.

Great Ninja war packs provide 5,8, or 10.

Monthly sign in-2 (5 and 17 days)

Monthly event can provide up to 10. (Except in Feburary, since no extra days will be present.)

Survival's lv 12 and breakthrough can give 1 at a time

You can get 2 from 3k frag point refine, 8 from 15k. But you could also end up with summon scrolls. 4/16 respectively.

Ninja test rarely gives 1.

Various events, such as Lunatic Halloween (up to 5 scrolls) are a good source.

Sage world can give as little as .3 if you're on the winning side.

Weekend events can give some to. (3 if you're lucky enough for TI one, 1 from chase mission, a total of .6 from ninja puzzle. 1-6 very rarely in dice...*sigh*)