First: It's foreword. (Lots of people spell it wrong.)
Second, there are lots of ways of contacting people who might be appropriate. Depends on your topic, of course. Your best bet is usually someone with impressive expertise-- a university professor, a doctor affiliated with a university or well-known hospital, a fellow writer who's written a lot in your field, etc. Or a celebrity who's been through what you're writing about.
Once you've come up with a wish list, basically, you just set out to contact each person (one at a time, so you don't get into the awkward situation of having someone say yes and then having to "un-ask" other people). It's usually not hard to track people down--
For authors, first check Google to see if the person has a site with contact info. If not, write to them in care of their most recent publisher.
For doctors, professors, etc., just call their work to ask for the appropriate address to send a letter.
See if you can meet anyone in person: look for conferences, book signings, seminars, etc. where you could meet with people who'd be appropriate for a foreword or blurbs.
For celebrities, it can take a little more work to find out who the person's publicist or agent is, but there are several sites with that kind of info.
www.imdb.com has a subscription-based feature (IMDB Pro, I think) that gives info about who represents actors and actresses. You can also call the SAG's actor hotline to find out the agent for any actor or actress in the Guild (free; you can ask for 3 at a time). I'm not up on which other sites are good, though-- anyone know?
So, once you get contact info, you write a personalized letter explaining who you are, what you're doing, what your book is about, why you're approaching him or her, etc. and offering to send the manuscript if there's any interest. Make sure to mention that reading the manuscript does not obligate the person in any way.
I'll add to this that I've never tried to get anyone to write a foreword for a book that's not under contract. However, I know that some people at least try to get the commitment beforehand, because that can help to convince a publisher to take on the book (if they know you've lined up someone impressive for the foreword). The manuscript had darn well better be in fantastic shape before you show it to anyone, though. You don't want to blow your shot at a great foreword because the book wasn't edited or ready to be read yet. And the problem with that is that nonfiction is generally sold based on a book proposal, not a finished manuscript... so it's hard to get anyone who you don't know personally to agree to write a foreword for a book that's not been written yet.