Premade books covers are great for self-published authors because they are so much more affordable than custom covers. There are many people who sell premade covers out on the big web and, unfortunately, so many of them are… unprofessional, to say the least. Never choose a premade cover unless you run it by an objective, honest person who will tell you if this cover matches the awesomeness of your book.
I haven’t used any premade covers for my books so far, but I like browsing through them for inspiration.
Here are some sites that I think offer professional designs that will hold their own in this competitive business: