You don't need a sponsor!
Citizen you do not need a sponsor to extend your WHV for another six months after your first 6 months are up, read below I got this info from the Japanese consulate general in Toronto website.
"Period of Stay
A period of stay of up to six months can be initially granted at the port of entry. This may be extended up to another six months by the Immigration Authorities in Japan."
make sure you go to the immigration authorities to extend your visa atleast 2 months before your first six months on your visa is about to expire. I didn't need a sponsor to get my visa extended for another 6 months, I just came back from doing a 1yr WHV. Canadians can do a WHV for up to a year, and then after that if you want to work legally in Japan you have to get your employer to sponsor you. but please contact a Japanese consulate in your city and they will give you more info on extending your visa.