Where to buy dry base rock online?

I was wondering is it a good idea to buy base rock online and if so, Is there any site I need to go to or does it matter? I have about 40lbs of rock in my 48 gallon tank now and it looks like I have a small mound in the tank. I would like to add more rock so I can aquascape a little better and have more real estate for future corals. Also is there a such thing as too much rock? I've heard that you want about a pound per gallon but I've also heard of some people having 3 pounds to the gallon.