We aren't allowed to post competeitor links.
Live rock varies immensely in price and quality, and the two are not always related. You can pay a lot for crap rock or a little for great rock.
If you need a small amount, I prefer to buy locally, since you can pick and choose what you want, and if you can transport it home submerged, it probably won't need re-curing.
As far as online options, google is a great starting place. Suffice to say, SWF.COM sells some great live rock. I haven't used it myself, but I've heard nothing but good things about it.