The best place to start researching is the Teen Vogue Handbook. It's a guide on how to begin your career in the fashion industry, and helps with preparation for such a profession.
There are also many sources for internships online.
My top 3 are:
In addition to these, I suggest making a twitter and following all your favorite companies. It may seem strange that a company would look for an intern off twitter, but its the reality!
I found an internship off of twitter because I followed my favorite store.
Also check on the companies websites for an intern coordinator.
My final tip is: if you want to work for a magazine, and have a favorite editor, email the editor personally, (and professionally!) and you never know where you will end up. I suggest not emailing Anna_Wintour@condenast,.com, as it will automatically become spam.
Or find out who the editors assistant is. As long as you remain professional, you have nothing to lose.
(Next week will be about how to write a professional email)