Bear in mind that I'm talking about nonfiction books here (since I have no experience with fiction): when we are in the manuscript editing stages (the authors are under contract, have submitted a mansucript, and we're all rolling along), we send out review copies of the manuscript and blurb requests to selected authors, experts in the field, or other appropriate potential blurbers. I always ask my authors either for a list of potential contacts we can send the manuscript to, or if they are comfortable and prefer it, to do it themselves. We've had great success obtaining comments from well-known experts simply by asking nicely. I emailed James McPherson, Pulitzer Prize-winning historian at Princeton and told him we had a new Civil War book for kids that I hoped he'd take a look at, and he was incredible--reviewed the entire thing, made some suggestions for revisions and additions, and gave us a really terrific blurb. Tom Wolfe and some pretty big Sports Illustrated guys helped us out on another book, too.
I'm not sure it matters if your blurbers are fiction authors or simply people who are in a position to comment appropriately on the book:scientists, politicians, historians, etc. are also really good sources.
Aruna, the reason your publisher may have used the same quotes for more than one book is that they could have gotten a bit jammed up and didn't get the new book out for blurbing in time before they needed to go to press. We've had that problem occasionally, and I've re-edited longer blurbs about an author to fit a second book when I didn't have a specific good quote ready for her new one. Publishers usually cover themselves by saying, "Praise for XX's writing" and then listing the earlier book title and the quote, or simply using a quote that comments on the author's general writing style. It's better to have a recycled quote than no quote at all.
For Jenna's latest book, she asked for--and received--terrific blurbs from all sorts of respected experts in the field, and had a response far faster than any we could have received. I'm sure we'll get some great reviews from the trades, too.
Sometimes authors reach a little too high for potential blurbers or they are looking in the wrong places: we had an author who wanted the head of the Metropolitan Museum of Art to endorse his book. Well, me too, but since neither one of us knew this person and the author's book wasn't especially relevant to anything pertaining to the Met, I wasn't going there.