They are extremely rare.
I did a search at Getty and iStock, only 12 images were returned. And none of them actually had a picture Mr. Jackson! http://www.istockphoto.com/file_search.php?text=michael+jackson&action=file
His official imagery will be controlled by the rights holders for decades to come.
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Your best bet is to find an amateur who took a picture of him on the street and negotiate some sort of use rights through them.
Oh, one last thought, his mugshot may be public domain. You need to verify that though. http://www.flickr.com/photos/medipixel/248470030/
