Google is your friend. If you are dealing with infringement by a print publication you will likely be able to locate the publisher's registered office easily enough. Try to find out who the editor or publisher is so you can pursue the named individual.

Where web infringements are concerned, it is often easy to trace the domain owner using a WHOIS lookup. There is a WHOIS tool right here.

We recommend all communications with infringers be in writing and that you post your letter using Recorded Delivery. It is amazing how often such letters otherwise allegedly go astray. Email is also susceptible to loss and denial. Requiring a receipt only works if the recipient allows it.