While B&H Photo and Adorama are pretty great, I disagree that they are the only two to deal with.
Take your time and search around for good deals. Like several have mentioned, Amazon is a great source and generally has some of the cheapest prices. Especially if you are looking for inexpensive camera accessories, like flashes, remotes, battery grips, and lighting accessories. There are a lot of decent cheaper options out there over the brand-name models; just do your research and consider the reviews carefully.
Sometimes you can find great deals on Craigslist too, but you have to be more careful about scams.