I believe it is possible to get hold of some small e-ink displays but I don’t know if you can get hold of just the e-ink itself.
I see the displays used for shelf displays in the local supermarket every now and again. I’m always tempted to just add one to my shopping and see what the checkout staff do. 
ETA: I have found some displays on Alibaba for $6 plus $11 shipping. https://m.alibaba.com/product/60419260217/High-Quality-GooDisplay-display-panel-1.html if you’re interested in testing these I can order a few.