What is the best way to find someone's mobile number? or many mobile phone numbers?

Do a google search for cell phone number directory and try the ones
that allow you to search for a specific name.  I believe all of these
are opt-in though.

At least in the US, I think publishing a list such as this may be
illegal.  If not illegal, I think it would end up resulting in some
sort of civil action as people received and paid for unwanted phone
calls.