Where can I buy the same নেকলেস as Bella ??

I've seen it on the Warner Bros website as well as on E-bay several times. As for me, I made my own.

on ebay link

amazon.com

type: "birdskull neclace" অথবা "Bellatrix neclace"

I got mine at hot topic a while back.

Try amazon, ebay, the warner brothers site. Just check around online.