Can your library get you items through an inter-library loan? That’s one way to go, if you plan ahead so your books are available close enough to when you need them. That doesn’t always work, though, if you are going to need something for longer than you can borrow it/renew it, but we’ve mostly had success with that.
Also, if you can find out where there are any really good used curriculum fairs (or library book sales), they are usually worth the drive even if you have to go farther than you would travel to shop. You might want to find a homeschool group online forum that is local to a nearby city and ask the parents there what sales they would recommend. You can also ask them (or ask on this forum) if you are looking for some specific items.
Finally, we purchase things through Amazon, eBay, and AbeBooks online, or at homeschoolclassifieds.com. Some members here will post when they list books for sale on their blogs, such as the wonderful, huge list mentioned by Melissa in this thread: https://simplycharlottemason.com/scmforum/topic/buying-used-books-2/
HTH,
Sue