Technical new brake light bulb

[tj130289]

my dash is telling me that my drivers side brake light has failed i have read threads with people putting the wrong bulb in so i just wanna know what the right one is and where is the best place to get it?
thanks

 

[Most Easterly Pandas]

It's a standard 21watt stop/indicator/fog/reverse bulb

Motorfactors be cheapest.