A curated archive dedicated solely to mockups. Unlike generic stock sites, every submission here is vetted for quality.
Free is fantastic, but it has limits. If you are a professional designer or a high-volume author (5+ books per year), you may hit a wall with free resources. book mockup free
Excellent source for clean, professional book, magazine, and stationary mockups. Resource Boy Book Mockups A curated archive dedicated solely to mockups
They offer a "10 Free Book Mockups" pack that is a staple for many indie authors. book mockup free