Hi, and welcome to Mobileread! The ebookwise is a great device, and we're still waiting for some bright tech company to come up with a new reader that reads a few more filetypes and has smaller batteries, but has all the other great features of the ebookwise.
For starters, free ebooks:
Mobileread forums host a large collection of IMP (ebookwise) ebooks, at
https://www.mobileread.com/forums/ebooks.php?forumid=153. You can sort by genre or just scroll through by author.
The
Baen free library has its books in several formats, and IMP is one of them.
Not free ebooks:
Fictionwise has a lot of books for sale in "multiformat," which includes IMP as one of its options. And while
Smashwords books aren't available in IMP directly, they are available in HTML which can be run through the conversion program to be readable. Smashwords is a self-publishing ebook site; it has both free and paid ebooks.