You can get a custom self inking stamp for free right now. I think these stamps are really nice and there are lots of options for these. You do have to cover shipping charges and that worked out to $5.99 for me.
They are great for addresses, labeling baked goods, bills and more. Another bonus is right now you can choose from the premium designs for free as well. And you can also upload a photo or logo for free.
I really like the idea of using these for books. It might be an idea for teacher appreciation day which is coming in May. I think this would be a really useful gift. You can also see more teacher appreciation ideas here.