Buddhist Conch Shell Horn Question

[FSrobin]

I will be in Bangkok on my way to Bhutan for three days starting April 12. I have promised a friend (Domkhar Lam) that I will bring him a conch shell horn known as a shank I believe. Is there a market in Bangkok that I can go to find such a thing? it does not need to be already fashioned into a horn as he can do that later, but it does need to be a real shell used by Buddhist for this purpose, not a fake shank like a lightening welk.

Thank you for your help