Patrice Clay's 'Find People' function might have limited reach. At the end of the day, Clay doesn't connected directly to LinkedIn, but rather, they rely on multiple data providers when it comes to contact information.
Secondly, Find People works better companies that have a linkedin presence. If the list of companies you have is often smaller companies, Clay might not have them.
Personally, I prefer to create a claygent that goes to the web to find the right people, and return to you the full name + title, and you go ahead to find their email addresses afterwards with a waterfall method.
Claygent can search the web in real-life and return you more update to date information,