I usually use a few places depending on what I am looking for. livestock: liveaquaria.com, liverocks.com, and saltwaterfish.com. Dry goods: diyreefs.com, ebay.com, and aquariumguys.com (just found them). Good price on salt and substrate. You could always visit the sponsors on this site as well. Using them, you might get a discount.
So far, all these places have been great.