The Math department does not hire TAs from outside the department.
The Math department hires homework graders each quarter during the academic year (not Summer quarter). To be eligible to grade for a course you must have taken the course at Stanford and received a grade of A- or better, or have equivalent AP credit for calculus courses, as well as have taken at least one Math Department course at Stanford. Grading assignments are made a week or two before the start of each quarter (in early December for winter quarter). If you are interested in being hired as a grader, please fill out the webform which will put you on the list of interested graders. If we have a position available we will contact you.