Access to panels is possible, you just need to find the right distributor. But getting the display is just a small part of the story. You need to build your driving electronics and figure out how to drive the display. 3-4 inches are also usually much simpler to drive, you can use a TCON and there's no waveform management, etc...
The fact that this is much harder than it seems (hardware is non-mainstream, access to documentation is limited, support is very hard to get, displays need special driving chips) actually prompted us to provide a development platform for E Ink. It took us years to figure everything out and for somebody wanting to prototype something this is a show stopper.
As far as pricing goes, the 6" panels are priced approachable, while the large displays still need to hit the mass production price points. Obviously if you buy from us in bulk we can provide you much better pricing than a price for a development kit, but E Ink is not cheap.