IMO, The best teaching letters come from senior faculty members who have supervised you as a teacher and have sat in on several of your classes to observe your teaching. At many schools, it is standard procedure for this to happen. The letter should also discuss student evaluations, including numerical scores and some positive comments made by students on their evaluations. If this doesn't happen automatically at your school, then you should ask a faculty member to visit your class. This could be your advisor, or the faculty member in charge of the course you're teaching, or someone else on the faculty who you feel comfortable approaching. I would be very skeptical, for a variety of reasons, of an application whose only teaching letter(s) are from undergraduate(s). Indeed, any class is likely to contain at least a few students who are enthusiastic about the teacher, and a few who have a very negative opinion, so a couple of positive letters from handpicked student don't mean that much.