Where is the idea?

I have "friend" who is starting his first startup,
As a long time wantrepreneurs he always thought that by the time he will finally start his company, he will have a killer idea, but it didn't really happened..

After I read famous startup success I could see it is sometimes the case,

I really like Elad Gil's suggestions:

• "Start a company.  Don't start an idea" 
•  "Focus on a great market, not a great idea"

Than how do you go about finding great market?