1. Captives from other regions that Rome had conquered during their expansion.
2. Children of slaves, which was more beneficial because they didn't need to buy them.
3. Taking abandoned and exposed children from the streets.
4. Trading with other groups of people from other regions such as, Western Europe, India, & etc.
5. Some would voluntarily sell themselves or their children to slavery, often dye to poverty and fear of starvation, both of which would be less of a problem after becoming a slave.