Six sigma training is offered in many ways. Here are several that I have experienced first-hand.
Local colleges often offer courses. I personally took Six Sigma Greenbelt classes at my local community college years ago.It was held over the course of a semester and required regular class attendance, homework, a final exam and a semester-long cap stone project.
On The Job Training
Many companies ask Six Sigma Black Belts to train high-potential candidates in their organization to be Six Sigma Green Belts. I have personally coached candidates to receive their Six Sigma Greenbelt.
Several reputable companies offer on-line courses. The best blend text books, off-line video, and on-line instructor-led classes. I personally took the Villanova Six Sigma Black Belt course and it offered all of those methods.