Personally, for the last few years I've been buying Optimum Nutrition in bulk (10lb bag) off Amazon here:
It is not the cheapest price you can find, but it isn't far off either, especially considering a few different factors:
1) You KNOW with 100% certainty these guys are solid quality. Time and time again when tested, their protein comes out as one of the top in quality and purity
2) Buying the bulk 10lb bag like this brings the price down quite a bit. So yes, the price tag looks pretty hefty, but think about it - 10 freaking POUNDS!!! Imo, it's well worth it, and the price is very competitive
3) Buying a 10lb bag means less having to think about this. That 10lb bag will last you quite a while, so you don't have to go shopping for your whey again anytime soon
4) Theirs mixes VERY easy, which can't be said about all brands that are out there
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To add, I also buy their weight gainer bag as well. 12lb bag, and again a very competitive price. Plus, it includes creatine in it, along with a bunch of other vitamins and minerals. Since you're looking for whey, I won't say any more than that on the gainer
On a side note, for in case you need whey asap at any point and don't have Amazon Prime... in a situation like that, I would typically go with Walmart and grab their Body Fortress whey protein. Good price, mixes fairly easy as well.