I recently started reading the Re:Zero light novel in Japanese. It's freakin' tough. The word choice is crazy sometimes (from the perspective of someone who usually only practices Japanese with manga). Part of the problem is certainly that novels just naturally use more descriptive words than manga and I'm not used to that, but even so I'm finding it tougher to read than even most seinen manga. Still, my e-reader's built-in dictionary makes the lookups quick enough that it's an overall enjoyable experience. Just slow. I mean, I'm 2 hours into the first volume and only 17% of the way through it!

The e-reader has been indispensable for this, hence my question: "Are there any websites that sell light novels in Japanese in epub format?"

Comparable formats would be fine. The important part is the ability to open the file in the koreader app and use koreader's built-in dictionary functionality.