[GeoffR]

I usually find the publisher's website is the best place to get covers. Some publishers have high resolution covers available for download from their website even though they put crappy low resolution versions (or no cover at all) in the ebook itself.

Edit: But you might need to look at the publisher's site in a different country, depending where you are. For example I have found that Random House UK doesn't provide high resolution covers, but Random House Australia does, for exactly the same book.