In my opinion, you would do best to re-think the idea of getting your certificate online. First, though there may be a worthwhile course online, nothing can replace the hands on experience you will get from enrolling in a program that enables you to work closely with well experienced teachers. Also, if your intention is to work in the US, you must realize that most employers will look very closely at your qualifications. When doing so, I doubt if they will take very seriously any TESOL certificate that you got online.
However, having said that, I do imagine that because the demand for qualified ESL/EFL instructors has yet to peak, there are a few courses offered online by respectable colleges or universities that will require your presence on-campus as a part of the curriculum - at least for a few days.
If you do choose to take an online course, my advice is to be very discerning before doing so. But again, nothing can match the intensity, thoroughness or respectability of an in-house program.
Good luck!