It’s getting harder and harder to get used textbooks because of the “digital codes” that colleges require, but you can still find used college textbooks for $2-3 that work perfectly for credit by exam or homeschool! Most of the CLEP books my teens used in high school were purchased from thrift stores, hand-me-downs, or shared with friends. Prep books often have practice tests, which are vital for a good layered approach. I just ran a query for CLEP, DSST, and Advanced Placement books as well as regular college textbooks that could be used for homeschool curriculum over at Textbooks(.)com and there were many!