What about self-sponsorship?
method 1 is how most people do it:
you come to japan on a work visa (if you're a teacher) for a year. during that year you make your own school or contract yourself out to other places who pay you for your work. when you go to renew your visa you apply for it as self sponsored. this allows you to quit any full time job and removes the burden of having a company sponsor your visa. this requires a minimum monthly income of between 150,000 and 250,000yen a month (provable) to be approved.