If you're going to kill what's on live rock, I'd go with base rock. Base rock will become live rock in a short amount of time and has no hitchikers.
We've had great base rock from reefcleaners.org. $95 for 50 pounds and it's very porus so gives a good surface area for your beneficial bacteria. Also, you can tell them what size tank it's for and request rock shapes that you'd prefer so you'll get what you like that will fit well for your aquarium in the comments section of the order form.
Here's a picture of our 45 gallon with the base rock that we had orderd from them. We had just upgraded our 10 gal
. into the 45 gallon, so the live rock in the picture is from the 10 gallon.