Looking at their site Rosetta doesn't appear to offer their books directly. Clicking the buy button provides a popup with links to some stores that sell them.
Kobo appears to only have five of Rosetta's Churchill titles available (for US buyers anyway) which would of course work on your Aura HD. B&N sells most of them and Amazon has all of them, but books from either would need DRM, if any is used, removed and in the case of Amazon conversion to ePub. They don't appear to sell on Diesel, eBooks.com, Sony or OmniLit at all.
Another alternative would be to use their contact form and see if/when they'll be selling the rest on Kobo...
http://www.rosettabooks.com/contact/contact-form/