Apart from Korea and Japan, Taiwan is another favourite holiday destination for Singaporeans. Although you cannot purchase a Taiwan prepaid data SIM card (unlimited data) online prior to arrival, you can easily purchase a minimum 3-day package at the Airport for NT$300 (approximately S$12).
The following contains the pricing table (that is only applicable to Airport store) from the 3 Telcos at Taiwan Taoyuan International Airport. Please take note that they are only open from 8am onwards, so you may have to wait for a while if you are taking a red-eye flight. And as advised by a Taiwanese friend, Chunghwa Telecom has the best coverage.
1) Chunghwa Telecom
2) Far EastTone
3) Taiwan Mobile