Mountain Rose Herbs looks like a great source. A few things you might consider when comparing prices of prepared green drink mixes and purchasing bulk powders is:
Are the powders made from powdered whole product or powdered JUICEs of the product?
How are the powders prepared? What Temperatures are they dried at?
My personal choice at the moment is Uri Internationals 'The Feast', but I don't think it has seaweeds in it. It is made from organic or transition (tested non-toxic) juice powders dried very quickly at temperatures never reaching over 100 degrees F. and they are a big supplier of Barley Grass juice powder to lots of other products and companies since the 80's.