1. Write to the cultural attache of the Mexican embassy in your country, and ask if there are any shops with masa harina in China. They probably have some supply of it themselves. If no joy, carry on with the embassies of other south american states. You probably have some Cuban community over there, students etc. Try and contact them.
2. Similarly, you will have african students/businesmen. In africa a flour made from WHITE maize is produced. I SUSPECT it is like massa harina. But I know very little about the cooking of black africa.
3. You can make sustitute this way:
Use 2 oz wheat flour and 2 oz polenta (fine) plus an egg. Knead together and add liquid to make a pliable dough. Then make into tortillas.
Alternatively, you can add more liquid/milk, and fry them like pancakes. This basically makes a corn flavoured wheat tortilla.
But it is not as good as the real thing.