New York and Boston. That's why they controlled the cotton trade. They had the ships, and the Southerners were prevented from using foreign ships because of the "Navigation Act of 1817."
But with a huge difference in the taxes from New York to Southern ports, the trade traffic would have moved. It would just take longer, but few people of that era would ignore a 30 to 40% increase in profits.
Now what were the two largest imported commodities brought to the United States at that time?
You tell me. I don't remember, and that's if I ever bothered to learn it.
And who is using more of those two things, the north or the south?
The nature of the question infers that it was the North, but the thing you won't explain is where did their money come from to buy those imported goods?
Imports are paid for by exports, and the South produced 73% of those. If you are just now figuring out that the North had developed a way to grab most of that money, then you are starting to grasp how they could pay for imports.
So what are stopping the south from building their own ships? Developing their own port facilities? Why was poor little snowflake secesh at the mercy of the dastardly New Yorkers for a hundred years?