You don't do that. That's your publisher's PR department's job, not yours.
Actually, I contacted a couple of authors that ended up giving us blurbs. My agent asked me to feel around and see if I could get a couple, and the authors I contacted ended up fulfilling their promises. My editor did the lion's share of the work, though, by sending out the manuscript and so on. My Dutch publisher passed the book on to David Baldacci, and we got his blurb that way. (True story.)
HOWEVER, no one's going to give you a blurb unless the MS is sent to them by an editor. Don't send your book to someone in the hope that they're going to read it before it has been purchased, because unless they're a very nice human being and/or have a lot of free time on their hands, they're not going to.