Where can I buy cheap books??

I’m a nanny and my boss (single father) is refusing to buy his daughter any books. Someone on here told me about an app called hoopla which is an amazing app that I got her to use however, there’s a series of books that she really wants that the app doesn’t have

It’s called “the babysitters club” and there’s 7 books in the series. She has 6 and 7 so she wants 1-5 but I can’t find them anywhere for a good price. I’ve even tried ThriftBooks which is where I buy most of my college textbooks for SUPER cheap... however they are sold out of the 1-4 package box and only have the 1-7 package box but that is more expensive then buying it on amazon for some reason.

Does anyone have an idea of where I can get these books for less then $40?

I’ve already looked at the library and they don’t have them either.