I buy pure cold processed micro filtered whey isolate. (no flavoring, just 'instatized' for use in the food industry). It comes in 20kg bags (44lbs).
Im on my second bag now. Sucks to say I am not sure if I will be able to get more after this. Got it for only $6/lb.
The same stuff can be bought packaged from NOW in 10lb bags though. I will be going to that after this runs out. (just about 10$/lb.) Still WAY WAY cheaper than most wheys...even cheap junk when you look at the cost per gram of protein you get.
This stuff mixes very light and almost unnoticeable in milk. I can add 2 scoops to 8oz of milk and not notice any increase in viscosity. I add my own flavorings when I want it....