First, if you are a member of the SCCA you are welcome to volunteer any club events that are held at the track.
Otherwise, you should consider joining United States Auto Club (USAC) who provide volunteer marshals the rest of the time. There is a fee associated with joining USAC on event basis however it is significantly smaller than that of SCCA.
You may also consider joining the IMS payroll as an employee. It is my understanding that they provide workers for events that are short on flaggers or rescue personnel, however your job will most likely involve track maintenance or security services, etc.