How to book in advance a bus from Vladivostok to Harbin?

Hello!

I'm a french guy finishing his roadtrip in Russia from Moscow to Vladivostok on September 6th, 2015. Here in Paris they ask for the chinese visa a proof of my entry and exit by a plane/train/bus ticket. Booking a plane in advance from Vladivostok to Beijing for example would be no problem and I could have my visa. But because I would like to cross the boarder by bus, I need to book this famous Vladivostok-HB bus via internet and in advance!

Does someone here have an idea how I could do this?

Thanks a lot in advance! :)

Pierre