The first place to start looking is
Google
Your second step is to use Google to research specific reps. You
can also type in “sales agent”.
As with any agent or representative you need to be very, very wary
of paying them up front. On the other hand, like any agent or
representative if they are agreeing to work strictly on commission
you will need to have a product they fully believe in. And those
are very few and far between. After all, working for six to
eighteen months repping a film can cost them a LOT of money.
So most will charge a fee. When I said be wary I did not mean all
who charge an upfront fee are scams - just to do your research and
do it well. Chances are you will spend as much time and effort
vetting agents or reps as you did making your movie.
I know there are bad reps out there but most of the horror stories
you read about are from disgruntled filmmakers who simply did
not have a movie the rep could place. That's a bitter pill to swallow
and few filmmakers can take the blame themselves.