You would prepare the same way for an advertised post. Target specific organizations and call them to ask the name of the hiring manager to address your cover letter to. Some websites will list contact information.
Ensure that your cover letter is concise. Get to the point in the first paragraph, as in: "I'm interested in a registered nurse position at xxx. This is what I have to offer, based on your organization's needs."
Look up the values and mission statement and work from there. Your letter needs to be specific about what you can offer THEM.
Make sure the cover letter is no longer than four concise paragraphs, otherwise you'll lose the reader's interest.