The books listed on page 4 of week-plan are called the "Primary" resources and thus are in-print and will meet the learning objectives. We do not recommend you purchase the books on page 5 of your week-plan. They are there for the purpose of using what may be owned by many homeschoolers, or you can access through your public library. These may or may not seamlessly meet the learning objectives.
Our sister company, Bookshelf Central, carries all of the Primary resources and provides one-stop shopping for other subjects as well. There, you can sort books by unit, learning level, and subject so that you have a handy book list. You'll also see how many weeks a given book is used so that you can make the best buying decision possible. Prices at this family-owned business are very competitive with other major retailers, especially after you put all of the titles in your cart. Additionally, they have a Used Book Program and are developing their inventory of ebooks.
Of course, you are welcome to shop with a vendor of your choosing or use your public library.