I can answer the second question but not the first. Most slaves were children born to slave parents. The child of a slave mother is the property of the mother's owner (even if the father was free). The second most common source of slaves was people selling themselves or their children into slavery. People might sell themselves into slavery because a slave's lifestyle was, in some cases, not so bad. Many poor people abandoned children they could not care for, and some of these were taken as slaves. A very important source of slaves was those captured in war. However, most war-slaves were taken on the fringes of the expansive Empire. While very important symbolically, this source provided a minority of slaves. It was thought that certain financial matters were best delegated to slaves or freed slaves of the family. In certain rare cases, people who were not in desperate straits would sell themselves into slavery to a rich and powerful master in the hopes of securing a high position.