Anything except a true Kindle is going to be hard on the eyes long term because they are backlit. Kindle is the only one with the cool ink technology. I read constantly--like 3 books a week--and for a brief period when my Kindle died and I had to borrow another e-reader, I had a headache the size of Alaska. Most times one can go with a substitute or a generic, but in this case, Kindle is really the only option. IMHO.
You can buy them online at Amazon and it takes a week to get to your EPS account and then here.