there are a number of North retailers in Perth, so depending on where you are try the closest one of these:
Australian Kiteboarding school (pinnaroo point)
SOS (freemantle)
WA surf (safety bay)
Kite addiction (myaree)
or if your out of town:
Geraldton surf and sail (geraldton ) or
Delta designs (margaret river)
the part is readily available. when you replace it you should also check the other line that passes through the bar as they tend to wear at a similar rate and it is advisable to replace both at the same time.