Creative Fabrica is my go-to site for interiors. At this time they have over 4000 products under KDP Interiors. Whatever products you pick make sure you make it yours by modifying it so it’s unique. I have a monthly plan so I got access to all of them but if you don’t want a monthly plan you can buy them individually. They often run good deals around holidays for both plans and products.
Tangent Templates and Book Bolt are two others that are awesome. Book bolt you can access some of their interior generators for free and more for a monthly fee. Tangent Templates you have to pay upfront, but it’s a one-time fee. They both offer some awesome services besides interiors for your KDP business like browser extensions that help you enter your book details. I currently only have Tangent Templates but you really can’t go wrong with either service.
Create Your Own
Creating Interiors is pretty easy. I most often create my own, I often use ones on Amazon or Creative Fabrica as inspiration. My go-to tool for this is Affinity Publisher. If you want free Canva and Google Slides work great and I recommend starting with them and you can always upgrade to something later when you start earning money.
Whatever you choose just get started creating products and make sure you make it your own.
Some of the above links are affiliate links.