Thrift stores! Once you are back at school send home Scholastic Book orders and use your bonus points for free books. I also used the friends of the library frequently. I often had a list of topics/authors and the lady that ran it was nice enough to save the books I wanted for me.
I second thrift stores, asking for donations on social media especially now that people may have done a massive cleaning, once open--libraries used book sales. Once you get going in your room, yes-Scholastic will be helpful. Perhaps there are also teachers on your staff who are not coming back who have books they are going to part with.
I've posted on my personal Facebook page and on Nextdoor that I am looking for books for my classroom. I've had friends and complete strangers donate books for my classroom. I've also looked at the free section of those pages and have found people giving away free books! Garage sales and library sales are another great spot.