I can recomend Live Rock UK They are good on price and fantastic on delivery also nice rock full of life.
I have also just ordered 20kg from HERE They have good feedback about the quality, and they have been very helpful so far with advise etc. Rock comes middle of next week so I'll let you know then
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I got mine from liverock uk as well. It looked pretty bare at the time but has developed really nicely and there was a lot more life in there then I originally thought.
How much rock are you after? If its a lot then it might be worth getting most of it from a cheap online source like the one above and then pick a few choice bits from your LFS.
Looked great when it arrived and I've had lots of odd bits n bobs pop up in the last 6 months. My only complaint that the 'nano pack' pieces weren't as nano as I would have hoped, but thats nothing that couldnt be sorted with a hammer and a chisel.