As a follow-up to my original post about MedicAlert:
In the end, I got a simple metal MedicAlert necklace (with medallion slightly larger than a US quarter coin) that mentions my having been a (left-side) living kidney donor, and mentioning to avoid NSAIDS, nephrotoxic medication and contrast dyes, as well as my asthma. I also received a wallet card with some more details (such as my name, blood type, wife's contact details, family doctor's address and phone, and a phone number [with the ability to call "reverse-charges", world-wide] for their central data base to learn more medical details). As I recall, I filled out a questionnaire, and their staff decided which details to inscribe on the device, and which to print on the wallet card.
In the US, MedicAlert can be reached via:
http://www.medicalert.org/ They have different arrangements: you can just get a Medic Alert necklace, bracelet, etc., or, for more money, subscribe to the full service in which they maintain a file (as up-to-date as you keep it) with your fuller medical details. They may also have family and multi-year subscriptions.
If you're out of the US, and the US MedicAlert doesn't serve your country, you might try the UK branch:
http://medicalert.org.uk/info@medicalert.org.uk
Note: unlike the US branch, the UK one does not allow you just to get the device alone. You have to subscribe to the service, too, which can cost 30 pounds/year (there may be different plans).
Another option is to try to order just a (stand-alone) device from Amazon. Some are pre-printed with messages like "Diabetes" or "Penicillin allergy", but some let you custom-order your own text engraved. When I was looking last spring, I remember seeing an inexpensive dogtag-like device that came with up to 18 lines of custom info engraved.
Finally: there is a range of MedicAlert devices, ranging from the plain-vanilla metal ones (necklace, bracelet, etc.) to fancier, costlier ones that are designed to be more jewelry-like. The necklaces have a very wide range of lengths of chains.
Be well, Snoopy