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Best I've found. Normally I just get it at my local Vitamin Shoppe for $40.
For pure whey isolate or rarer proteins such as micellar casein, hemp, etc. you can also try True Protein | High Quality Nutrition and Protein Supplements
Microfiltered whey isolate is $9.11 per pound and ion exchange whey isolate (same stuff as Isopure) is $9.40 per lb. You have to pay a bit extra for flavoring, adding digestive enzymes, or a vitamin/mineral blend. Personally, I'd just buy it plain and put it in a blender with other things (mocha powder & milk, fruits, fruit juice, whatever) for flavoring. Basically, if you go to True Protein you can get 3 lbs of the same kind of protein as Isopure for $28.20 plus shipping and handling. You just won't get their vitamin/mineral blend and flavoring, which are things you can really get from your groceries and a multivitamin anyway.