It’s extremely difficult to find free generators for puzzles that allow you to use them commercially, which is understandable. However, at the time of writing, you can create them for free here. The free versions of your puzzles, however, won’t be as good quality, so you may want to consider upgrading quite cheaply.
They also have links to Windows software for a one-off payment, so you can create as many puzzles as you want. These are great for activity book fillers, and your puzzles will be unique. You can download countless puzzles from Creative Fabrica (affiliate link), but remember, other people will be using the same ones in all probability.
You can download puzzle templates from Canva, but you will need to upgrade to use most of them. Here are some more puzzle generators, but please make sure you have the rights to anything you generate before you use it:
Printable Mazes – But there is no reply to any questions about commercial use, so please use them with caution.
You can always search for free generators, but please read any terms and conditions for commercial use very carefully!
You will need to be able to download them as high-quality PNG or PDF files to add them to your books, and don’t forget to download the solutions to add to the back of your puzzle books!