How to get feedback when creating a new SaaS?

There is no easy way except for being familiar with that vertical and those communities. If you're building a SaaS in a space that you're passionate about, you would already familiarize yourself with the competitors in that space and their limitations (and maybe that's why you're building your SaaS). If you're in that space, chances are you would already be talking to potential users of your SaaS, maybe understanding their pain points with current solutions (which could help shape your product roadmap). If you're doing all of these, then getting feedback is relatively simple; you already are in the community that may be searching for an alternative, and when you show them your SaaS, they, as target potential users of your app, should have really insightful feedback.