ISBN numbers are required for all textbooks in order to be professionally printed and distributed. Many self-publishing companies will take care of acquiring an ISBN number for you as part of their service. However, if you are printing your textbook yourself, you will need to acquire an ISBN number on your own.
There are 3 ways to acquire an ISBN number:
Apply online at www.isbn.org.
Complete and submit the ISBN Application Form by mail, fax, or email.
Contact your local ISBN Agency.
Once you have applied for and been assigned an ISBN, you will need to include the ISBN number on your copyright page, as well as on the back cover and spine of your book.
For more detailed instructions, please see this blog post: http://blog.bookbaby.com/2015/02/how-to-get-an-isbn-number-for-your-book/.