How Do I Get U2 tickets?

I have a friend who is absolutely obsessed with the band U2.  She has been a fan ever since she was a little girl and has yet to see them live in concert.  I told her that she may not have that long before they call it quits and should see them as soon as possible.  She is taking my advice and looking for a place to get a good deal on a few U2 tickets.  I told her that I would do some research myself and see what I could come up with.  I am hoping between the two of us she sees her dreams come true.