If there is an art college nearby, put a notice up on the notice boards. Ask a painting or drawing prof to ask the class if anyone wants to do a piece for an ebook cover.
Photography clubs. Lots of them take photos. Amateur clubs. Look them up.
Stock photos.
Or, find a picture you like on Flickr and ask the owner if you can purchase the right for use for your book. People ask me for the right to my Flickr pictures about once a month but I say no. Ususally they want to use them for free in a calendar or book...yea right...pay up I say.