Is there a source for learning the names of NEW agents in the Hollywood
Film business?

I realize that agents are very secretive once they are established,
but I was wondering, if, when they first become agents there are
certain media or columns where they announce becoming agents or
joining an agency.

The two major entertainment trade publications on top of agency news: 
DAILY VARIETY, WEEKLY VARIETY, DAILY HOLLYWOOD REPORTER & WEEKLY
HOLLYWOOD REPORTER.  They regularly report who has been hired, who has
been fired, who has been acquired.  Or promoted.   Usually the
agencies themselves send out press releases announcing their new "ten
percenters."   Both publiciations have online versions, too, but
they're not free, unfortunately.