1) Talk to business owners and inquire about their frequent problems, biggest expenses, etc.
Maybe your business can help.
2) You hear from more than one person that no X software, app, etc exists.
Maybe you could make a product that fills the needs.
3) Maybe you dealt with a business and said to yourself "I could do way better than them."
4) Maybe by doing some of the things at this page, you come up with a BIG idea.