The best source is likely to be a very wide ranging diet of whole veg etc, but a practical level many of us do not achieve that so there is a school of thought that green supplements are useful to widen our diet etc.
There are lots
http://www.google.co.uk/search?clien...=Google+Search
Green Frog is one of a number I have used.
http://www.vitacost.com/productResul...t=green%20frog