There are loads of ways to get the seals over time, and as Kimchi said, it'd take an average of 2 months for 210.

I'll list a few methods.

Chase Missions (1 per weekend)

Survival (1-2 per day, I've gotten 2 in a day before)

Fragment Refining (Chance for 2 in the 3k point one, chance for 8 in the 15k point one)

Various Events (The Star Wheel has a chance to drop one)

Ninja Test (Getting all 10 provides a chance for 1 scroll, it happened to me twice, one time being yesterday)

Great Ninja War Packs-(5/8/10, depending on the pack)

Monthly Login-(2 per month, 1 on 5 days, 1 on 17)

Monthly event (10 possible per month, 8 for Febuary, 2 per week, which equates to 300 points per week, 1500 points per month...except for next month/Non leap year Feb)

Monthly rewards in Arena (0.5 for student, total of 60 frags, or 6 scrolls per month, if you can get to 6 paths)

Seal Rebates are also handy, as they'll give you seals as you use them. So try using your seals around then. There's 2 different seal rebates. Select Treasures, and just seal rebates. Select Treasures (Such as what we saw last week or something) give more.

Fighting Matsuri Challenge gives more the farther you manage to go.

Sage Worlds

Wanted Ninja

Treasure hinting can give 0.5.