Out of curiosity, where do you think Southerners could have purchased more slaves from? After all, the importation of slaves had been illegal for decades before the Civil War. Perhaps a small number could have been smuggled into the country, but they would have been few in number and likely quite conspicuous.
To this day you can buy slaves. If you have money someone will provide them. But if you are naive paulite nitwit you wouldn’t be able to grasp that.
The importation of drugs have been illegal for decades and we see that hasn’t stopped anyone from bringing in tons of the stuff.
Supply alway crops up to match market demand.
The south relied very heavily on manual labor. If you could have waved a wand and taken those laborers (i.e., slaves) away in one short day, the next day the people in the south who had owned slaves the day before would have either had to sit by and watch their economy crash, or would have to find new workers.
Assuming that the plan to actually purchase freedom for the slaves had worked, the “need” for new slaves, and the money to buy them would have both existed in the south.
When the alternative would have been economic devastation, the supply would have come into being one way or the other.