Microsoft has published a list of ebook stores that sell .lit format books at the bottom of the page here: <
http://www.microsoft.com/reader/us/shop/publishers.aspx>. I've mostly looked at BooksOnBoard myself. It appears that Microsoft has its own ebook .lit search engine too, with direct links to the individual stores that carry the titles searched for. You may want to try an inexpensive book first to test out the process and make sure everything works right on your computer. You could also try converting one of the free .lit books from right here on Mobileread, but these books don't have copy protection, so you wouldn't be able to test the DRM-stripping tools which you need for store-bought books.
I'll get around to trying it out myself one of these days here. I may crack and try it before I'm done with my writing for the month.