How to get the Flightticket early on?

Hey there!
I am currently studying in Shanghai but I only have a single-entry visum.
Now at the end of the semester, I plan to got to Seoul for 7 days. So my route would be:

Shanghai --> Seoul (05.01.2019)
Seoul --> Shanghai (13.01.2019)
++++ 144h Visum in Shanghai ++++
Shanghai --> Manila (19.01.2019)

In order to get the 144h visum in Shanghai I already need the flight ticket with seat number and date when coming from Seoul, right?
How do I get the Ticket in advance?

Thank you for your help!