I've done both of these when I was fishing for SaaS business opportunities.

To narrow down the niche market, identify the key departments, type of problem, and industry verticals of target companies. The users of your solution would benefit either from $$$ or productivity point of view.

Then to find early adopters, find out who is attempting to make their own solutions already (in this case, consult the consultants who deal with your problem). You can even buy PPC ads on Facebook or work with their other vendors to see which customers are motivated and proactively looking for a solution to their problem.

Hope this helps. If you want more clarifications, let's set up a call.