Where to gather white coral rocks for lava graffiti?

I noticed there were a few jellies that are from Hawaii and I’m sure that there are others that are familiar with these waters, so I thought I’d throw this question out here.

Family is vacationing in Hawaii and after seeing all of the lava graffiti (messages “written” with white coral rock placed on top of black lava) I’m quite determined to add my own piece. Except I’m a dorky tourist and I’m not sure exactly where the best place to get these rocks?

I obviously don’t want to take from other people’s work. (Though the crumbly illegible ones are tempting.)

We’re on the Big Island (in Holualoa). We’re thinking about doing it somewhere along the 19?

Any other tips also appreciated!
Thanks, island jellies!