Ask people what they use Excel for and convert those into SaaS apps (you won't believe the things people do with Excel - I've seen Financial models running in Excel!). Even if you know nothing about an industry, this can be a quick conversation starter "do you use Excel often? what do you use it for?"

Try old school, boring, slow moving industries for low hanging fruits - manufacturing, leasing heavy equipment and even agriculture/animal raising in some cases. These have actual time saving requirements than building yet another project management application