Canon doesn't make printheads commercially available for that model. According to a Canon rep I know, all their recent models have "lifetime" printheads that should last for life of the unit under normal usage (life of unit is determined to be about 4-5 years by most manufacturers). Sometimes you can order a part like that through a Canon rep. However, the factory quoted $79.95 for an S300 printhead when I was just curious, and that printer retailed for $99.95! Go figure....
It surprises me that you already need a new printhead. Did you buy your printer with an extended warranty? Most printers are made to be throw-away now and they'd usually just give you a replacement unit under warranty because it's cheaper than actually fixing it.