Kobo and BAM will both give you an ePub so would probably be easiest. If you want to read it in Marvin you'll have to remove the DRM to do so. The Chapters-Indigo option will just take you to Kobo to buy.
B&N makes it harder to download an actual file separate from using their app as a reader.
Amazon should work fine, but then you'll have to both remove the DRM and convert (which should work fine).
You can also get the DRM'd ePub from Google Play
https://play.google.com/store/books/...d=wnK7eIQ2IjUC
Of course you can use the store of your choices iOS reading app to read without any DRM removal or conversion necessary.