Hello

I am an Australian with a Celta certificate and good conversational Mandarin. I would like to work In Shanghai however I do not have a BA degree is it possible to get a Z visa with out this?
I have heard that people with out a BA are able to work for private tuition companies but I have read that ainvitation letter from a relevant Department of Chinese Government or a government-authorized company is neccesary. Would a private english teaching company be able to provide this?