It depends on the country or region but popular way is to hit on the local chamber of commerce or department of commerce.
However, the most potential way is the traditional one, "Friends introduction".
In fact, ocean forwarding is the difficult field to make differences to others. If you have a strong advantage such as logistics, warehouse or good rate for such "After clearance service", it is easier. But friends introduction is the shortcut to gain trust and make difference to others.
I hope this hint supports you.