The mirrors in Software Sources & proxima_centauri's website are the mirrors of the small Linux Mint repository.
I think you want a mirror for the Ubuntu repository instead: the Ubuntu repository contains some 99% of the packages.
You can go to the Others tab, and there edit Ubuntu and Recommended Updates.
The mirror addresses can be found here: https://launchpad.net/ubuntu/+archivemirrors
There are quite some Taiwanese mirrors in the list.
Registered Linux User #528502
Feel free to correct me if I'm trying to write in Spanish, French or German.