The Government of Canada’s official website to apply for an eTA. It only costs $7 CAD. Most eTA applications are approved within minutes of applying.
New entry requirement now in effect: visa-exempt foreign nationals who fly to, or transit through a Canadian airport, need an Electronic Travel Authorization (eTA). Exceptions include U.S. citizens and travellers with a valid Canadian visa. Canadian citizens, including dual citizens, and Canadian permanent residents cannot apply for an eTA.
Visa-exempt travellers do not need an eTA when entering Canada by land (e.g. car or bus) or sea (e.g. cruise ship).
An eTA can be valid for up to five years or until your passport expires. If you get a new passport, you will need to get a new eTA. With a valid eTA you can travel to Canada as often as you want for short stays (normally for up to six months at a time).
Information taken on Immigration Canada website: http://www.cic.gc.ca/english/visit/eta.asp