I bought some descaler (also for kettles, irons, etc) from a hardware/DIY store on 2/F Jelita shopping centre but I guess you can find it everywhere. I think the brand's Rayon.
I found this "recipe" using vinegar on the internet:
Descaler
# One part vinegar with two parts water is all you need to clean your kettle and iron. Kettle - pour into the kettle bring to a boil, rinse well.
# Iron - Put some in the resevoir, let it stand for 30 minutes, rinse well.