Slaves did not go and "find" jobs outside of their plantations. They were used to work off of the plantation during war time to build fortresses, carry the supplies or dig latrines. Because there was a possiblity that they could escape, they were watched much closer than they normally would have been on the home plantation.
Some slaves were allowed to work for others away from the plantations they were enslaved on. My ancestors were enslaved in TN on a plantation called Wessyngton. It was owned by the Washington family, relatives of President Washington. In 1860 it was the largest tobacco plantation in America, contained 13,100 acres and 274 enslaved African Americans Slaves were assigned a task on the plantation and once this was completed the slaves could raise their own tobacco crops and they were paid by the owner. He also paid them for any work done during their time off such as Sunday and Christmas. All slave owners did not have enough work to have their slaves occupied year round and often hired their slaves to others. Skilled slaves such as blacksmiths, coopers, brickmasons, and carpenters often worked for others. In some cases they were allowed to keep some of the money they earned. This is how some slaves purchased their freedom.