Do you have data that supports this conclusion?
Three pieces. The first are figures listed in "Lifeline of the Confederacy: Blockade Running During the Civil War" by Stephen Wise. In one of his appendix he lists the net amount of revenue collected at each Customs House and lists a Congressional report as his source. According to his figures, the total net tariff revenue from the 11 busiest Southern ports in the year prior to the rebellion was $2,855,496.15. The net tariff revenue from the three busiest Northern ports was $42,551,216.87. That almost a 20 to 1 margin and means that 94% of tariff revenue was collected in Northern ports.
Second figure is from Alexander Stephens' speech to the Georgia legislature where he notes that the North accounts for three quarters of all overseas business, but Southerners hold most of the foreign embassy's.
Finally there is Lincolns December 1864 annual message to Congress where he lists the tariff revenue for FY 1863 as $102,136,152.99. That is without Southern consumers and with the Morrill Tariff. If the South did consume almost all of imports prior to the rebellion
Taken all together it's clear that the overwhelming majority of imports prior to the rebellion were consumed by Northerners. A minimum of 75% and a maximum of 96%.