What they said, I usually change plugs around 160k km/100k miles. I use the same ngk plugs that it came with new. Iridium plugs will be far more expensive than regular copper ones but they last a long time, why buy cheap ones unless you're getting rid of the car soon.
As far as ordering online over there, I can't help you there, try the googles.
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View my fuel log 2014 Mirage SE wussie cvt edition. 1.2 automatic: 37.7 mpg (US) ... 16.0 km/L ... 6.2 L/100 km ... 45.3 mpg (Imp)