There are a lot of rules about sponsoring visas. It can be tough, but not impossible, if you have a good immigration lawyer who can walk you through the process.
Some agencies will charge and arm and a leg. I was quoted close to 300,000 yen for the paperwork to file for my perminant visa, from a large firm. I did some research online and found a guy who did it for 40,000 yen for me. He is trustworthy and very dedicated. I have used his services successfully for 3 visas and he is in the process of sponsoring an English teacher for my school now.
Here is his info:
Say, Janica Sims recommended you :-)