Two approaches I've taken: What are the jobs in my industry that everyone does but no one enjoys? I ran a digital agency for a while. No one wanted to do maintenance work on a site, or security work. Or low budget work. We sat down and figured out more efficient ways to solve this problem and found people willing to refer the work into us.

What are the things in my company that suck to deal with? We sat down and listed out all the annoying things that we have to deal with on a day to day basis. Poorly run meetings, endless approvals on certain projects, an inflexible CMS...

Plenty of problems out there, just need to look.

One more approach: find a customer set you want to work with and grill them on their problems. What does a day in the life of X look like? What is the most painful thing about doing x? Why is that? What ways have you tried to solve that problem? Ideally x is a very boring industry or role.