Since I am an engineering student and don’t have an advisor in the economics department, who can I ask if I have questions regarding economics courses?

A great source of information about economics courses are your economics professors and teaching assistants. They may not know the specifics of the requirements of your degree but they do know a lot about the content of the courses offered at Columbia.
On the Sample Programs page of this website you will find courses grouped by interest to help you select your elective courses. Finally, if you are interested in pursuing a Ph.D. in economics, then you should consult with the director of undergraduate studies as well as read the section on graduate school in this FAQ.