Chamorro people used giant clam or basalt blades on their tools.

Since the same resource was available throught the South Pacific, it was used in those regions as well.

There were conch shell axes used along the Gulf coast and in the Flordia Keys.  And there are axes that look similar to your ulu, just with a longer handle.  Don't know what kind of shell it was, check out the Key Marcos exepidition.

  Yes, the large clam shells were what was used for axes, but anything thick would work.  These are pretty thin, I would think you could maybe use it on wood somewhat but the edge would chip and break and flake off.  The thing about these is that if you aren't careful shaping them they will flake, as in a layer will peel off one side.  Makes it not much good for making jewelery.

  Something you could try is at seafood restaurants, they probably throw out all kinds of clam, mussel and oyster shells, you might just get some for free by asking.