A fantastic not for profit organization operated for the benefit of teachers. It's no secret that teachers often pay for teaching supplies out of their own pocket. Treasures 4 Teachers accepts donations and resells the items to teachers at remarkably low prices.
Be aware that shopping here requires proof of employment as a teacher and payment of a $35 annual membership fee. They give tours to those considering membership, so you're not forced to commit before knowing if they sell what you need. Trust me. Once you see the inventory and prices, you'll sign up.
They are a bit strict about only letting members go into the shopping area. They actually have a waiting room of sorts for non members.
The store includes a "fill-a-bag for $5" area, a free area, an individually priced area... Lots of books, games, office supplies, teaching resources... Just keep in mind there's a randomness to the inventory linked to what is donated. The store is remarkably well organized given that fact. I'd suggest all teachers drop in for a tour and see for themselves. Not a teacher? Stop in for a tour as well. See what types of donations they'll accept from you.