Hi Sabine,
I actually mentioned the use of olive leaf extract in an article that I wrote for the Olive press magazine, you should be able to see it online, it is part of an article on olive oil and I gave a recipe for olive leaf tea!!! I thought it was crazy that people were burning all that brash when there are companies charging a lot for the extract. I have purchased the extract before when it was recommended to me, it was in a pill form. I will look into this and let you know what companies are selling it. Of course it should be organic, many Spanish farmers spray large amounts of herbicide beneath the trees to kill weeds, that is their choice but no good if you are wanting to sell the leaves as a health product.
I'll post again when I get more information.