Learn to find where to check:
First find local sources such as:
library used book stores
What do you like and buy?
What does your family and friends like and buy?
Get products from different niches.
People often forget what is unique about their part of the world. They see the things around them everyday, walk past it, until it becomes part of the background noise, lost.
Stop. Look around you. What is there that no one else has?
I have traveled and I know that every area of the world has its own unique flavor, its own food, clothes, spices, toys, and they have access to items that no one else has. These items are also cheaper there than anywhere. What is common with you is uncommon a thousand miles away.
So stop for a moment and think about the area you are in, what is unique about your area. What can you get there that others in the world cannot?
Those items are worth more in other parts of the world than where you are, and they will pay more than you currently pay. These items cheap to you can be sold for more money to others elsewhere in the world.
Stop the self editing for a moment, stop the how to questions, and just think of the item. What value does it have for others?