Summary: Read S-1 forms for your business plan ideas.
When a company sells stock for the first time, it must file a s-1 form with the SEC. This form explains how the company will make money - its business plan!
Companies can cite anything from identifying a competitive edge in their field, a market inefficiency they will exploit, an opportunity to reduce costs in an industry where this is a problem (like Amazon and storeless internet retailing!), or otherwise explain their plan.
Quickest way to do this:
Go to EDGAR (the SEC database) and
1. Search Most Recent Filings
2. Limit to S-1 Form
3. Avoid the S-1/A (amendment forms) and read S-1s all day.
Now, one thing you can also do is go to the advanced search screen and type in a keyword (insurance, computers, etc.) and s-1. This should get you closer to browsing S-1 plans in your field.