These sites offer quality pre-designed ebook covers:

Literra Designs: covers starting at just $35. Literra keeps costs low by using free stock images. You can customize the cover text for free. Back-of-the book and spine designs cost extra.

Beetiful.com is much pricier, at $165 per cover, but the covers tend to look a bit more professional. If you’re serious about marketing your work, I recommend Beetiful.

Fostering-success.com has pre-designed covers starting at $99, plus an additional $85 for full wrap. Don’t let the weird website name turn you off. They have some decent covers at a mid-range price.

CCRbookcoverdesign skews more toward sci-fi and horror. If genre is your thing, these could work. Very affordably priced at $45-$75. (The primary image for this post is from CCR)

Damonza.com offers very good-looking, unique, artistically inclined covers, but at a heftier price than the others–$195. If you don’t mind spending more, this site is worth checking out. Unfortunately, some of the covers, while arresting, have titles that aren’t as readable as you might like them to be.

Goonwrite.com has covers starting at $30. The brightness of the images definitely gives the covers a pre-packaged look, but that doesn’t mean they’re not eye-catching. If you scroll down you’ll find searchable genre categories on the left hand side of the page.

Author Marketing Club  features covers by a number of designers. You choose the design, and AMC hooks you up with the designer. Covers are affordably priced, although some look a bit too formulaic. AMC offers is a membership of $105 per year, which gives you free access to all of their ebook cover designs.