Most of the slaves in Roman civilization were obtained through war. As a reward for victory, Roman soldiers captured millions of prisoners. They turned defeated soldiers into slaves. Sometimes they would keep them captive, sometimes they would kill them.
In addition, pirates captured large numbers of people and sold them as slaves. There were hundreds of slave markets under Rome. The main market was on the island of Delos in the Aegean Sea; up to 10,000 men and women were bought and sold in this market daily. The largest export of slaves from here was to Italy.
Also, at that time, people could sell their children. Again, creditors could accept debtors as slaves. Of course, no one could sell themselves as slaves! This was considered fraud in Roman society.
777.677 SRY