I would approach the math department at your school and see if there is a professor there who is willing to take you as an independent study student for one course. You would be wise to lay out a pretty detailed plan/proposal prior to having this talk with the math department. In other words put together a proposal that includes everything you would normally find on the syllabus, including what text book, what chapters/ sections will be included in the course, what exercises you will do. Also include a detailed timeline / schedule for all of these things. You want to show that you are willing and able to follow through to the end and that you will not be much of an additional burden on the prof. It would be well to model your proposal after the syllabus of from an actual class taught recently at your school. Good luck.