Start by calling the schools in your area. Ask for the person in charge of supplies and find out if the school is getting rid of any textbooks. Most schools are more than happy to give these books and materials away. You can even get workbooks for every subject.
Check your local newspaper for yard sales. Especially at the beginning and the end of the school year, teachers and homeschoolers are cleaning out their supplies and will be selling (or giving away) textbooks and workbooks for a reasonable price.
Call your local churches and ask if they know of any programs that offer homeschool resources. Many churches keep track of homeschool events and activities.
Go to book fairs. Some communities have homeschool book fairs once or twice a year and this a great place to meet other homeschoolers and pick up needed supplies.
Hit the Internet for resources galore! You can find resources for Christian-based as well as traditional homeschooling at the websites listed in the Resources section below.