If you want your book to look good, then avoid asking a friend to do it, unless they are a graphic designer.
Depending on how much you want to spend, I can highly recommend TLC Graphics. They are outstanding and the results are excellent.http://www.tlcgraphics.com/
However, if you are looking for a good job done, without having to pay for the excellence, then try Freelancer.com.au I'm not sure if it is only based in Australia (or that's just the site I get sent to), but it's a global site. You can find book cover designers there (and I've seen some really good covers done through freelancers hired there), or you can post a job for freelancers to bid on. Either way, you have a great range of people available to work with you. Obviously, check their ratings/portfolio before proceeding.
Hope that helps.