The local public library can be your best friend. Take your kids a lot, so that they see it as a positive, fun place! Take them to story times and activities at the library as well as at the bookstore -- In our area, there are many public libraries (large library system) and the various locations, none very far away, host a LOT of things for kids: Story times, crafts geared to their age groups, games, movies, card game groups, readiing groups for older kids, visits from authors, pajama parties, much more. Get online with your library system and see what's out there.
If you get them used to the idea of borrowing books from a library, then when they are older and have full access to a school library, they will be much more interested in and eager about seeing all the school library has to offer them.
We buy books when the price is right or when we know DD (now a teen) is going to read and re-read and re-read again. There are some series like that, for her, back then when she was young and now as a teen. But the ability to borrow books also gives your kids the ability to taste and try new things, even if those new types of books don't turn out to be favorites.
There are also heaps of books available second-hand at many thrift shops, often including copies of popular books in excellent shape. If it weren't for thrift shops and libraries we'd spend way too much on books -- and we do still buy books via Amazon and at the bookstore. But I really have to recommend teaching your kids to think the library is a huge and frequent treat!
When they're in elementary (or now, if they already are), get to know the school librarian yourself. Volunteer to shelve books or otherwise help the librarian during the day while the kids are in school. Our elementary librarian had a roster and parents could sign up to shelve books, sort and label new books, pull books to fill teacher requests, etc. It helped the school and it really, really taught me a ton about kids' books and what's out there that you might never hear about if you just look on Amazon and at bookstores!