Do they work? - I can only state anecdotal evidence, Munchkin coped much better with teething after she got one, she also became a sunnier child, you could tell when she hadn't been wearing it! Mr Man teethed with minimal fuss, has had amber beads on since he was 3 months (now 3yrs 1month!). Again if he hasn't had them on for a while we notice the difference in him, less calm. They aren't for chewing on, they are designed to be worn, the effects are alledgedly due to micro oils being absorbed by the skin.
I swear by them the difference in my children is impressive, I have ordered one for my next baby before it is even born!
Are they safe? - This call is really rather up to you. Medicinally they can't do any harm, the risks that are associated with them are the potential for strangulation and/or choking hazards. Reputable retailers necklaces are made so that the cord the beads are strung on will snap if it should become caught in something (I can attest to this, Munchkin threw herself off my lap and the beads snagged in my fingers as I grabbed her, cord gave way, didn't even mark her neck).
They are also strung so there is a knot between each bead so should they break a maximum of one bead can escape. The idea is they wear it under their clothes so is unlikely to get caught anyway. So there is the potential of a slight risk, only you can decide whether to risk it. Though chances are you/other responsible adult will be close by so unlikely LO would manage to get bead into mouth without you noticing anyway!
Where is a reputable online shop to get one? - as MTM says a few stores have stopped trading (due to Europe/Trading Standards treating us all like mindless children), however I have just bought one for my bump from
http://www.amber-teething-necklaces.co.uk/ Good Baltic amber and nice quality, lots of choice of designs too!
Sorry is long, hope it helps