This is actually very common and can be (relatively) easy and cheap to find. Just search eBay using the right title.
Worn in: Nightmare in Silver
Topshop Embossed Headchain
We actually ID'd it before it reached Topshop's site, and it completely sold out within 12 hours. It is the Topshop Embossed Head Chain. For the episode, the top chain was removed (which is very easy because it is just held on with two jump rings). The elastic was also removed and replaced with hair clips, and the pendant itself is held on with some kind of body glue.
Worn in: Robots of Sherwood
Reiss Marissa Green Skull Embellished Brooch
Link to original listing here: http://www.reiss.com/us/womens/womens-sale/marissa/green/
Worn in: Listen
Moon Wooden Collar Clips by Ladybird Likes on Etsy
Still sometimes stocked on Etsy. Link: https://www.etsy.com/listing/113563459/moon-wooden-collar-clips?ref=hp_mod_rf