There are many reasons that ingredient prices vary between retailers.
It depends on whether or not the retailer buys in bulk, sells in bulk, buys older hops, malts, additives, etc..., and their relative overhead.
Most of the retailers on here are going to both sell quality ingredients and have very fair prices. Also, the customer service is going to be superior since most of them are brewing fanatics as well.
For hops and malts, look at farmhousebrewingsupply.com.
For kits, Austin Homebrew Supply is a great resource.
For equipment and DME & LME, I like Northern Brewer.
Yeast is pretty price competitive no matter where you look, so I like to tack it on to wherever I order my malt from to save the additional shipping.
Cheers!