Well this is a very valid point these days number of stores available dealing with organic foods and especialy wheatgrass powder is available by every Organic food store. I am using wheatgrass powder for almost a year and it has proved effective for me. I recommend you to check it from UK organic food providers. There are lots of them providing the best certified foods online, the one I am using is BodyMe.
You can give a look on the products of BodyMe Store UK