Edit: At least in the US, elephant ivory is NOT illegal to own, or even sell domestically, as long as it was either aquired legally or predates the ban, so your entire premise is incorrect, see https://www.fws.gov/guidance/sites/guidance/files/documents/...
Other sources from long extinct sources like as a sibling noted, mammoths, are not. It’s not even THAT expensive. For instance, a guitar nut in mammoth ivory is about $30 vs $5 for “bone” or $10 for synthetic/graphite