Book Bodies and Covers for a whole range of sizes are available from the KDP website. You may need to register to get a login, but that’s not a problem. It’s worth it – well, it’s free actually. The page is here. The action is simple. You select your book dimensions and download the appropriate template.
Everything You Need is on KDP or Lulu.com, or IngramSpark.
They all have most excellent guides as well.
Lulu is another very good place to build your books and covers. Their Template section is here. You will need a login.