I typically browse Newegg and Amazon, as they offer fast shipping. Lenovo's manufacturer page offers very good pricing on their Y series right now as part of a weekly deal, but they are known for shipping delays so avoid if that's important to you. Otherwise, I've seen more than a few people recommend xotic for their customization, selection, and competitive prices, though I've never shopped there myself (I am planning on it in the future though). I would steer clear of ibuypower, they gave my family nothing but trouble when we had their models, and the customer support was by far the worst I've ever experienced. Additionally I noticed problems with their build qualities on their desktops with each model we received (we got 3 at once...