GenWhy, anaphylaxis is the end result of an allergic reaction.... I'm allergic to penicillin, anaphylactic shock as a result. Rather unpleasant experience, isn't it?
Anyhoo, ack to the question:
you cannot buy Medic Alert bracelets over the counter or from other sites, you need your doctor to sign off on the medical details section of the form. They're instantly recognised by the medical profession, so they are worthwhile.
The tag part of the bracelets have a brief statement of your medical condition (diabetes, epilepsy, anaphylaxis, etc) and your rego number, as well as the phone number for Medic Alert Australia. MA has a 24 hour service, and they keep your medical details on record so they can provide information to the hospital etc, straight away. You fill out your membership form annually, and update your health information when you do (or straight away if something changes).