You can obtain the best weapons in Monster Hunter Stories through various ways. One common method is by completing quests. Some quests will reward you with weapon blueprints or even the actual weapons. Another way is by gathering materials from monsters. For example, if you want a weapon made from a certain monster part, you'll need to hunt that monster and collect the part.
To get the best weapons in Monster Hunter Stories, exploration is key. You need to search for rare materials in different areas of the game world. Some materials are only found in specific locations or during certain events. Also, you can trade with other characters in the game. They might have the materials or weapons you need. Additionally, leveling up your character and your monster companions can unlock access to better weapons.