Track down
Definition 1 of 2
Pro Tip 1/3Active Search
Implies a proactive and determined search, not just a casual look. 
She had to track down the missing files before the meeting.
Pro Tip 2/3Investigative Process
Often associated with detective work or solving a mystery. 
To solve the case, the detective tracked down every lead.
Pro Tip 3/3Persistence Required
Indicates that persistence is often needed to find something or someone. 
The journalist had to track down multiple sources for her article.