I'd first check your school's website - it might be listed there. If it is not, maybe it was sent home on a start of school handout? If you can't find it, you can always contact the school directly for it.
I agree with laurenf, the best way to locate your teacher's email address is through the school's website. Our school has a section for the instructional staff per grade and you are able to select that teacher to see what is happening in the classroom and his/her email address associated with the school is provided so that you can contact them. I would not expect to locate their private email address, but there should be an available address through the school.
First thing I would do is ask my child's teacher for their email address, if they refuse to give it to you then ask them what would be the best method of communicating with him or her. Explain to them why you would like to have their email address and let them know it would be strictly used for the purpose of knowing how your child is performing academically in school. A positive relationship/collaboration between the parents/teachers is very important in order for the child to succeed in school.