Use content that is not a textbook
While open educational resources can be a whole textbook, they can also be an openly licensed course, learning object, chapter, worksheet, assignment, or some combination of these things. What makes all of these things open educational resources is that they have been licensed to be easliy shared online at no cost, or in print at low cost.
While finding an entire open textbook to use to replace the more costly one you have been using sounds great, it may not be your best choice. Bringing together multiple resources that can achieve the same result is a common tactic in creating an open course. You will need to balance your pedagogy goals and needs with resource availability (there is not open content for every need - yet).
Start by looking at your learning objectives, and then search for content that supports student learning for each objective. You may end up with a variety of formats. Many open educational resources are licensed in such a way that taking a slice of a larger OER is not only acceptable, it is encouraged. Remember that there are different licensing options, and not all allow for remixing, so be sure to check to see what the license allows.
In short, if you find an open textbook that partway meets your needs, keep in mind that you have advance permission to keep what works and combine with other open content in order to create and share custom course materials.
When you change your course materials, follow your institution or department’s procedures to make the change official. The bookstore and other departments need to know so that they can stock the right materials and let students know about no-cost and low-cost course materials that are in use.