Where can I find the law regarding child support for children in college or age 18?

Your child support order, what you have to do and don't have to do should be spelled out in detail in the child support order,

The law varies by state within the US.

First you have to tell us where you are. ANY question on law needs to include your general locale as laws vary by area.

Most of the time this is a negotiation at the time the child support order is issued. The general standard is 18 or when they finish high school. Some orders are written for when they finish college. In the absence of a clear end date in the order, local law will apply.