This is a great question, thank you for asking it!

Firstly, there are two types of tumors that can occur in the brain: primary (a tumor that originates in the brain) and secondary (when the cancer has spread from somewhere else to the brain.

Most brain tumors in children are primary tumors whereas most brain tumors in adults are secondary.

You are right in saying that nerve cells cannot undergo mitosis cell division, but apart from neurons, there are other supporting cells such as various glial cells, blood vessel cells, pituitary gland cells, meningeal cells, and those of the nerve sheaths and skull, and other cells that are able to replicate in brains and so that is where the cancer comes from.

First, not all of the cells in the brain are nerve cells; second, and more importantly, nerve cells can't divide because the genes that enable cell division are turned off; they still have them. Becoming cancerous involves turning those genes back on.