fictionwise sells .imp books, look for the books that are listed as multi-format.
Any books that come in a regular txt file or a html file can be converted to a .imp file through the librarian software.
You can also convert books to .imp such as .lit and .mobi format. I prefer .lit if I need to reformat, as it generally comes out looking very good.
See wiki info at:
https://wiki.mobileread.com/wiki/IMP_software
Let us know if you have any more questions!
Amy