Textbook copies for Outreach Programs
Contact your local library and ask them to help, they may be able to access copies through inter-library loans. If you're located in the US or Turkey, it appears that this book is available via OverDrive (https://www.overdrive.com/media/96681/how-to-prove-it) , which is a commonly used eBook lending service. It's quite likely that you have a library nearby which offers OverDrive to their clients, in which case, you'd get free access with membership and so would your students. Good luck and keep up the good work :-)
ETA: I'm not affiliated with OverDrive in any way. Of course, there could be other eBook providers which have this book, just check with your library.