I still don’t know a good source for finding business problems to solve. You end up having to talk to people which is slow and won’t give you a large list. Ideally I’d want a huge list to pick from.
You can start with notes of problems you meet daily or ideas you come up with. If you want to pursue someone else ideas, most likely you won't have the determination to build things for years, fail and learn from these mistakes.
It takes time, tears and sweat and a lot of failures to actually build something successful, it won't be a one night sucess from idea taken from someone.