So while I am typing this I have Amazon open in another browser tab.
I hover over "Shop by Department"
I then select "Full Store Directory"
I clicked a random department, for this example I select "Kitchen & Dining"
On the left there are sub categories. I select "Cookware"
Again, on the left are sub-sub categories. I select "Fondue"
There is my niche. Fondue. If Amazon has a category for something then more than likely it is making sales, and is a niche worth looking into.
Of course I will now have to do a bit more research such as keywords and whatnot, but there is just one process for finding a niche.
Thats all I have to say, thanks for listening