Most of the kits my friend and I brew from come from Northern Brewer. Their quality is fairly consistent and we've done some good things straight from what they offer on their website. However, when buying online, do not opt for purchasing the yeast that way, as no matter the packing method the yeast doesn't quite stay cold enough in our experience. Buy that locally.
For the recommendations above, Winemaker's Shop and Gentiles are great bets. Winemaker's Shop primarily carries Wyeast, which is our preferred type. Gentiles carries White Labs, which also produces good results (though we prefer Wyeast) and might be a necessary replacement if the type of yeast you're trying to use isn't available at Winemaker's.
Gentiles definitely wins on convenience for the hours they're open. We once completely clogged a siphon with hops and had to make a last minute trip there at night to find a replacement. Both are good for supplies in general, and the staff at Winemaker's is hugely knowledgeable.