I'm running out of ideas on where to find books that don't cost "an arm and a leg".
I'd rather not do Amazon if I can help it.
I buy quite a few of our library books from https://abebooks.com.
Many have been hard cover, in really good shape, and priced as low as $3 or $4 plus shipping. And sometimes shipping is free if the book costs $4 or $5. I believe they are trustworthy and provide good service. All the books are rated as to condition, and soft or hard cover.
I joined CBD/Christian Book Distributors which gets you lots of extra sale catalogs, emails and free shipping options. Their prices are usually cheaper than Amazon. Also, LifeWay has some good online sales and free shipping options. I've also ordered from Alibris.
I just joined and am totally new to being a church librarian. But I am a public library director of a non-profit library operating on a small budget. I love this site Book Depot. I believe their primary audience is book stores that have bargain sections. But these are all new books that are overstock. Some prices are as cheap as 50 cents.
They do have minimums. I believe you have to spend $100 and buy 2 copies of each book. Or spend $250 to get just one copy of each book. But they do have several Christian publishers, including Thomas Nelson and Zondervan. https://www.bookdepot.com
Probably 90% of our church library books come from yard sales! I very rarely pay more than $1 for a book. I even have several groups & individuals that now reach out to me whenever they have good Christian books available for sale. It is a win/win for everyone! I only buy books that I know our patrons will read, that are in good condition and that meet our mission. It is a labor of love!
Find Christ Books (call them)
Answers in Genesis
Sometimes from Library and Educational Services
Majesty Music (they do carry some books also- not many)
I also check for recommendations, World Magazine, the blog of Pastor Tim Challies (Challies.com)- good book reviews.
I love to go to local library book sales. I've scored about 35 books in the past month and another library has a sale starting this Friday. We have a small church library and I have pictures of all the CF/YF and I have pictures of the books I've already bought and ones on my own bookshelf that will go to our Media Center when I read them. It helps me to not buy duplicates. But...sometimes one slips in, so I plan on keeping it along with some others to give away somewhere.