As mentioned searching the forums on here will give you as much info as you can process. Then just start doing some internet searching, and don't be afraid to look beyond the small handful of names that keep popping up.
List of English Schools and Academies in Japan | Teach English in Asia
That site lists a lot of schools you've never heard of but do you research! Some aren't even in business anymore.
Also I believe it's Jasonbvr on here has a great lengthy thread about the ins and outs of being and ALT