it depends, if you have little experience curing live rock or little patience I would suggest talking to a LFS and seeing if they have some TRUELY cured live rock or if they are willing to cure the live rock for you. This is easier but much more expensive. But on the other hand, you only need 10-15lbs so paying 3-4 dollars more for LFS's live rock isnt going to hurt you as bad as paying for 100lb of live rock. But like always, do some research about your LFS first.
Now if you do order online, then find the best quality LR, not the cheapest, since the cheapest is normally so bad that you wasted your money on dead rock. I would say that stewie and jasutton are both right depending on what you want. Most LR has been harvested many days/weeks before you recieve it so overnight isnt absolutely nessecary, but AVOID shipping that takes longer than 3 days. Overnight may give you better lifeforms if the rock is truely uncured and very 'fresh', but if it is older live rock that has been 'precuring" or sitting in a spray tank, then overnight isnt going to do you much good.
So LR is going to take 2+ weeks to cure and it is going to stink! so you will need a extra protein skimmer and alot of water changes (every 1-2days) so often it is smarter to get it from a LFS if you dont want to go through the trouble, but if you are willing to do the work, normally some online places have better quality stuff than the LFS.
In my case, I actually got my LR from Ebay. There was someone who was moving and needed to sell his tank, so I bought some of the live rock from him and it cost me 3.5 bucks per pound (including shipping) and since he live about 50 miles I was able to have UPS ground deliver next day, so the more you look the better deal you can find!