Here is a place that sells it:
You could cut it yourself with a band saw, if you have one.
I think the coralline will look cool after it starts growing on it.
I have sand. I tried CC, it got dirty and was a pain to clean, it will also limit your clean up crew and any other critters that like the sand. I think, if you are going with a small amount, like you said, it might still be relatively easy to keep white and nice looking.
Whatever you decide, pleas post pics.