Rescue the Juinchuriki, Nine Tails Invasion and Cat Quiz are all decent sources os sun scrolls even if you're f2p, and not raising your blood pressure either.

Decisive Bonds is an option, but very tough if you're weak.

The solution is simple and hard at the same time: you have to attend most of these events throughout the week and gather little by little. For example: even if you loose all your matches in matsuri, that's still around 200 sun scrolls.