where to get spark plug for spear & jackson petrol strimmer M28?

Hi I need a replacement spark plug for my spear & jackson petrol strimmer model M28 serial no 07 its 28cc app 0113-02 family M28A01 these are all the details I have for machine I dont have the spark plug as there was none in it when I acquired the strimmer hoping someone can help me out with a number for the spark plug or where to purchase it thanks