For modern hilts, there's a company that's making Alaskan ulus with "mermaid bone" aka Steller's Sea Cow bone.  As with fossil mammoth tusks, the material is available.  I don't know how far it ships, but I'd be unsurprised to see "mermaid bone" show up in any *modern* hilt.   
 
As for traditional hilts, that's a different issue entirely.   
 
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