Don’t underestimate the value of your local library (and their library network) when it comes to needing books for your curriculum. Even if there is a wait list, you can often delay certain parts of your curriculum until the book comes in, however in our experience, there usually isn’t much of a wait list.
Definitely take advantage of ordering, or at least researching, potential library books online. Then you can consider going and making a field trip of the library excursion, asking your children to help you find the books based on the Dewey Decimal System.
Don’t fall into the trap of buying all your homeschooling books, even if you are considering reselling them. And some libraries with a large homeschool presense either have a selection of study units available or are happy to order some of the common units in for their library inventory.