As a single mother, Do I need a consent letter to travel abroad with my child? If so, how do I get the consent letter?

I am a single mother and my child is 4 years old. I was never married to the father of my child. He abandoned the child when the child was about a year old. I have been the only person taking responsibility for the child. Thus, since he abandoned the child, my child bears my surname. The child's document like the NIN and international passport carries my surname. So I learned that I may need a consent letter to travel if I intend to travel with my child, how best can I handle this issue?