I've been thinking about making one, I make my own jewelry It's usually cheeper, it actually would be really easy, if you like to make jewelry, all you would have to do is go to a craft store and buy chain, clasps, and some jump rings (they are just the rings that hold charms on jewelry) depending on where you buy them they are very reasonably priced, and then look for similar charms in stores or on the Internet, I've never found any of my pentagrams in regular stores, a great place to buy **these types of charms (please read below)** is:
I may sound like I am repeating myself but **please people don't wear these types of things for the wrong reasons like to intimidate people or to make people think you worship Satan or anything like that, I believe people that do that offend wiccan and pagan religions (Wiccans and Pagans don't worship Satan)** I just want people to be aware of this, I'm sure your not going to wear it for those reasons though.
I bought mine on ebay. Just type in "mary winchester bracelet". It gave me three or four different variations [most obviously crudely fashioned home-made ones]
there were a couple of nice looking ones, better quality. I love mine, wear it eery day.