Mini-van - Bangkok to Laem Sok Pier

We will be traveling to Koh Kut (Koh Kood) in late March and will spend a couple of days in Koh Chang too. We've done Koh Chang before and I know about the mini-van from Suvarnabhumi all the way to Koh Chang. But this time we are arriving late at night in Bangkok and will need to start out early the next morning from wherever it is we will stay the night. I thought we would go to Koh Kut first for a couple of days and then to Koh Chang.

How do we find a mini-van to get to the pier at Laem Sok, Trat? Do we have to go to Victory Monument or someplace to get the mini-van? We will need to leave by 7:00.

Thanks in advance.