You can buy from any microstock agency such as shutterstock, istock, etc, that gives you royalty-free rights. Keep in mind that the images won't be exclusive - means others can buy an use - but you can modify them in the cover design. If you want exclusivity it becomes expensive.
DepositPhotos has a twice a year deal through AppSumo where you can get 100 credits for under 50cents apiece. The next one should be in May. Royalty free stock images for up to 500k uses
123rf has a standard license that includes a up to 500,000 copies of a cover. It also offers several different sizes and types of downloads. I buy credits vice the monthly subscription.
Comments
I don't use the AI at 123rf. I'm a bit disappointed that they went there, but everyone dud so I just don't use it.