I would like to hear any advice you/others have about finding a product in your niche to manufacture!
helps if you have breadth of knowledge and experience in a variety of fields also. interdisciplinary solutions still have plenty of untapped market potential
other than that there is no magic formula that i know of. its kind of one of those "if you have to ask..." type things.
For example, hang out at small-town airports or boat harbors, or with lawyers and doctors. If you can find some small part that would help them use their boat, then $300-500 for it might not even be an extreme price especially if they see you using it first.
(This is not an argument to go buy a boat and plane).