My suggestion is freepik, you can find a lot of (thousands of) designs readily available and suitable for your book in any category. Most of them can download at free of cost. But you need some Basic knowledge of Photoshop or Illustrator to edit the areas,change colors, add a custom field or image. Photoshop is very simple to use, a lot of tutorials are available on YouTube. If you have no idea about Photoshop, find a good designer in your locality or hire a designer from Envato market or Fiver (online). My suggestion is Envato.