You could also consider getting a working holiday visa, although you need some funds saved up. This will allow you to search for a job while there. Going on a tourist visa and jobsearching in Japan is also possible, but a little more risky because you’d have to find a job and get a visa within 90 days. Perfectly possible, but there’s a chance you might not find anything and have to go back home.
As for the job itself, I found my job on Craigslist. You obviously just have to be careful and use your common sense. I think GaijinPot was mentioned earlier, too, but it’s 95% teaching jobs.
If you are interested in teaching, Interac and Shane English School employ people from overseas and have a visa waiting for when you get here :) Best of luck.