I have a novelette I'm in the processing of finishing and knew what I wanted for a cover but thought it would be very difficult to find.
Actually, it turned out to be quite easy. I found a very simple cartoon figure at a stock image site and it cost all of $2.99. I could have stopped there but I decided to change the colors and add a background so there was quite a bit of time investing in Photoshopping to get it where it needed to be.
But I could have stopped there. So that's the advice of the day. Go to the stock photo sites and look around. If you find something that's close, you can look for something similar. Read the tags for the photos and see what words are used by the photographer or illustrator to describe the image. It'll give you hints as to what search terms to use.
There are hundreds of thousands of images out there and it can be tricky to find something usable but tags can be your friend. Try to be specific but broad when you search. Think of what are the qualities of the image and how you would describe it to someone else.
A Google or Bing search will give you a long list of sites like Shutterstock, Fotolia, Dreamstime, istockphoto. Getty is going to be too expensive but most of them charge under $20 for a single image.
It's always good to have a image with a sizable area of negative space. That means where there's not much going on. You can put your text there. I'm not against putting text on the image but there's the chance it won't show up very well.
This works pretty well. Of course it's the wrong orientation, you need an image that's vertical not horizontal, but it shows how you can put the text in a blank space and how it balances the heavy foreground image on the right side.
If you keep these points in mind, choosing an image will become much easier and will require less work on your part to get a nice cover for your book.