Although creating an Open Access course from scratch sounds daunting, you probably already have plenty of material that you can use. Start from the materials that you have already created for your courses -- lecture notes, tests, quizzes, assignments, etc.
Textbook repositories have varying requirements for accepting material.
Remember that if you share your work on multiple repositories, you will have to update it in multiple places when you make edits.
Additionally, keep in mind that when you post your work in an OER repository, it may be harvested by several OER Portals, so that will help distribute your work as well.
OER materials often don't have ISBNs, but authors can register self-published materials through Bowker if they wish. There is a fee.