Kobo and Pocketbooks are all Linux based.
Both quite easy to hack in.
If selecting a couple of generation older, then these used to have external micro sds.

Kobo even had internal sdcard for its rom. They are widely available and not very expensive too.
Pocketbook have their sdk put up in github it think.

(..even the Kindle ls all linux, but pita. to hack in)

And special mention:
BQ Cervantes. Linux/QT, Open Source.
Slightly expensive and bit rare to find.