You can certainly check the books out of the library, and even make substitutions. In the 106 Days you will need the Thorton Burgess books for sure. You can get by without the science experiement book (Sonya explains it enough in the text) and most of the things for the experiments are already on your shelves. It is okay to look at a subject and make substitutions. It will not harm your children. =)
You can take a look at the subjects and look around your area. Go on field trips to anywhere that will help show your children what you are talking about. We have lots of geology things here, so I added a bunch of field trips in on that subject. Using nature videos about places works too – both for science and nature study. We learn a lot about places when we can see it – and reality is that we are probably not going to South America or Africa soon. lol As we are studying botany, I have the Nature Study kind of focus on plants instead of insects (although we still look at those too and vice versa when studying insects – you get the idea) You can make this all work together naturally if you remember to think about it.
If you do the Outdoor Challenge for Nature Study and add living books in for science, you can use those listed on the free curriculum guide (lots of those are free in the public domain) or available for $1.50-3.00 on Amazon. We are pretty partial to Thornton Burgess. 😉 They are fun and the characters tie in from all the stories. My children will say things on our Nature Walks that I know they learned from Thorton Burgess books.
Here is another list of books that perhaps you could get ideas from: http://www.pennygardner.com/sciencebks.html
There are lots of books choices and lists and at first it seems overwhelming (I even dreamed about it when we first started using CM). I promise that it gets better. =)