Hacker News Reader

Replacement displays for ebook readers are not that expensive and someone has figured out how to drive them without the proprietary controller: http://essentialscrap.com/eink/

Wow those are super cheap.. I'm tempted to pick up one and I don't even have a use case yet.

This is still in kick-starter phase, but if you don't mind waiting a few months it has a lot of potential IMO:

https://www.crowdsupply.com/e-radionica/inkplate-6

4.2" (Which sounds like it's too small for your needs): https://www.adafruit.com/product/4142

Wow that one also has a 25s refresh rate

Buy an old amazon kindle for $20. You can root them and remote into them via ssh for display control

I believe Waveshare gets their panels from the same place good-display.com does. The Waveshare adapters work on good-display panels.