Unfortunately, it was most likely a custom made prop for the film, and not something you can buy from a store or online.
It was made to resemble the magnifying glass necklace Ash gives Linda in Evil Dead 1 & 2, although with a modified design with less emphasis on looking like a magnifying glass.
In the new film, it's meant to be a good luck charm made from wood (buckthorn) that David gives to his superstitious sister Mia.
It's possible someone may start producing a replica (licensed or not) eventually in which case ebay is probably your best bet.