I use protein direct (
http://www.proteindirect.com.au/ ). They were the cheapest non-dodgy looking place for the kind of sizes i needed (i dont plan to go through bags and bags of the stuff, theres a lot of evidence that says protein supplements either dont do much at all or the boost is kind of minmal, so i half the dose a lot of the time).
Careful of the ebay stuff, you have no idea what it could be loaded with or where it came from. Just sayin' screwing up your body will hurt you more than an extra $20 bucks. Really you have no idea what they could adulterate it with or any impurities in there. Remember, this is concentrated protein, they take the whey or milk or what ever their base is and concentrate it down, this also concentrates down all the other crap in there too...cholesterol (quite high in some proteins from whey for instance), fat, pesticides, heavy metals, ect.
Make sure if they provide labels to check how much protein is actually in the protein. Whatever is in the bag isn't 100% protein.
See:
Even in the natural/unflavoured whey the difference between the amount of powder in a scoop and the amount of protein in that is about 10 grams, thats pretty significant, its a 25% loss roughly, 25% of your money is being spent on filler or whatever other stuff is in there.
32.5g of protein in 41g. (~79%)No idea why chocolate takes up so much more room than the other flavours but the same principal applies here. In a 5kg bag of flavoured vs unflavoured, there'll always be more protein in the unflavoured one. I still got flavoured but chose vanilla because a) vanilla rocks and b) it had the least difference in protein content between it and the unflavoured variety.
33.1g of protein in 45.5g. (~73%)So, if one company is cheaper than another, it could be cheaper simply because theres more filler. You can pretty easily divide how much your paying by how many grams of protein are in the actual pack and this is what i did to compare. Its a shame not all companies publish things like this though.
All good with the protein direct stuff though, very cheap, literally melts into the water so you dont need to use a concrete mixer to stir it and no unwanted health impacts.
Here is also a good list -
http://whirlpool.net.au/wiki/where_to_buy_supplements