Hi Yui,
No need to apologize for asking questions! Im happy to help. As far as gathering materials:
1. When you revisit sewers and win against beasts they drop materials.
2. There is chance to also find material searching the sewers.
3. Visiting the shops in the forest hideout and Brookhaven they sell 3 different materials daily.
4. Using the wandering option after all story related wandering is done, you have a chance to run into the black market seller who will sell all materials types at a high price. (Added in newest patch 2.82.)
5. Winning battles against beasts, there is a low chance of finding an egg. Bring the egg to the bestiary and hatch it. Feeding it daily will increase its bond level. At higher bond level it will give you 1 material corresponding to it's type daily.
Let me know if you still have problems! Good luck!