My good friend Renee is doing covers via her website The Cover Counts.She's running a $50/cover special right now, which is very reasonable and mid-low for price ranges for the people I've seen in the cover space.
There's more involved in doing a cover for POD book, because you need not just front 5x8 cover art, but also spine and back. So be prepared for cover designers to quote you two prices - one for just a flat cover, something higher for the whole wrapper.
Very few artists I know will deliver you original work for $40 a pop, but most will happily make small changes for low $, or give you a few versions for the set price you negotiate up front.
One of the whole points of epublishing your own stories is so you don't HAVE to bend to the details of specific market requests - you put your cover on your book and self publish it on whatever platforms you decide - smashwords, barnes&noble, amazon being the top 3 (smashwords gets you into the Apple iBookstore, as well as onto the Sony Reader and Kobo platforms, which have large installed bases and international readers who are hungry for english-language content.) They don't specify anything about cover other than max filesize and certain dimensions (B&N needs one slightly longer as I recall.)
Good luck launching yourself down the epub route! Oh, and here is an example of one of my short stories that she did the cover for - Library of the Apocalypse (it's also available on Amazon and Smashwords.) I knew I wanted a picture of this library from Chicago (the Harold Washington Library in downtown Chicago, which plays a leading role in the story) but I wasn't sure how to find one. Renee at The Cover Counts found the image, then did some cool effects to it to give it a true apocalypse feel. We went back and forth on font until we were both satisfied. It's neat, it totally represents what I wanted for the story!