Where can I get the 144-hour visa free exemption, coming from Mongolia ?

Hello,
I plan to come back from Ulan Bataat to Beijing bus bus or by train. I will leave Beijing the next day by plane toward Geneva (through Dubai).
In both these cases (bus or train), where will I be able to ask for the 144-hour free exemption ? When the bus or train crosses the Mongolia-China border ? Or can it be obtained only at the airport ?