They’re off the main path of the game, but there are smaller islands in Dragon Quest Builders 2 that allow you to get infinite supply of certain materials by completing Scavenger Hunt challenges. These islands are called Explorer’s Shores are smaller locations that don’t have the lengthy story and quests on them, but they will give you a chance to score some of the more popular materials so that you’ll never have to farm them again.
The first two islands that you can access with hearts are Blossom Bay and Soggy Skerry. Each island can give you two infinite supplies of materials if you hunt down items. On Soggy Skerry you can get an infinite supply of wood and an infinite supply of dry grass by tracking down 16 items for each. On Blossom Bay you have to track down 15 items (for each) to get an infinite supply of cord and grass fibre.
To get access to the islands you are going to need to pay Captain Brownbeard a bevy of hearts to gain access, but it’s well worth it. The scavenger hunt challenges aren’t necessarily hard, and can be completed fairly quickly. Once on the island all you need to do is find each item in the list and then when you encounter it hit the prompt that says to “Check” the item. Doing so will tick the item off of your list.