Hi I'm Irish and I am living in Japan and trying to visit Guangzhou and Hong Kong for 3 days...

I live in Sendai now and I plan to go to China next month. Since I already booked my flight, can I go to a travel agency still to get my visa?? The travel agencies I've been to so far said I can't get one from them if I don't book with them. What are my other options? From reading these threads it seems I can't mail in my application form. Do I have to go down to Roppongi to apply in person? Would they process it the same day?