I suspect that labour as cheap, or cheaper, COULD have been acquired from various regions of South America, but THEY would have been free workers, able to come and go at will. The whole point of slavery, it seems to me, was to have the workers under complete control, to get the crops planted and harvested, and meanwhile to breed more workers who would then become a commodity to sell for even more profit.

That is, regrettably, a 21st century perspective on a 18th century issue. Yes, plantation owners in the American South and the West Indies could have obtained workers in Central and South America - except that there was already a labor shortage there, caused at least in part because much of the indigenous population population had been enslaved by the Spanish and died. Whether they would have been, if imported as workers further north, "free to come and go at will" is pure conjecture. Personally, I doubt it. The point of slavery, whether one is talking about Africans in Georgia or Jews in Rome, is to obtain affordable labor when workers are a scarce commodity. Human ownership is unnecessary for effective control of the labor force - just ask any black sharecropper in the South during the century after the Emancipation Proclamation or any Russian serf before 1917.