Buy from tea masters who grow and process your tea themselves. Why, because many tea companies have no idea where their tea leaves are from. Moreover, they put all these leaves together and process them in big tea factories. As a result, you will not be able to confirm if the leaves are from a natural tea farm (probably not) and the unique characteristics (terroir) of every single batch will just be wasted.
When the tea grower understands nature and takes care of his tea garden with respect, then they will never ever consider using chemicals to make their life easier or get more profit.