they're basically any large roundish smooth "bauble" on the end of a leather chord.....a necklace that will withstand yanking on by baby (and not hurt your neck). There are places that actually sell them as "nursing necklaces" but really, you can find something that fits the bill just about anywhere.....I'm thinking flea markets, craft sales or any of those jewellery booths that set up in malls etc.
It's basically something to redirect the baby's attention onto when their freehand is busy swiping at you or going for your hair - you can just gently guide their hand onto the stone instead......it was also suggested to me that I could try it for my DD to focus on instead of constantly whipping her head around to look at stuff and popping off
HTH