there's a list of online programs on this page: https://math.stackexchange.com/questions/734/online-math-degree-programs
you may need to use google to find current links for some of them.
most will allow you to take a few courses as a non-degree seeking student. there's not really any reason to do a whole online masters if you're considering grad school in math.
it won't be easy to get good letters of recommendation from online courses, though.