Walmart is currently having a sale where you can buy a $50 Gift Card for $35. I've never actually tested it but from what I can google online, it appears these will work for iBooks.
Dear Author has an entry saying it will and a couple of other places.
http://dearauthor.com/wordpress/2010...nd-other-apps/
AFAIK, it still isn't possible to remove iBook DRM, so only if you have an iThing to read on.