Possibly only from the shop you bought the machine from. These cheap mowers that are sold by B&Q or Lidls or wherever are a nightmare to get spares for. I can't remember where we found them when I worked in garden machinery. Challenge may be a rebadged Castel Garden or Stiga so trying someone that sells those could be worth a try. is the blade bent or does it just need a sharpen and balance? Again, a proper garden machinery centre is what you need.
Jules
July 2010