MOBI ebooks are sold by many ebook stores, but they are almost always selling ebooks they get from mobipocket.com. They are often selling them for less than mobipocket.com, so it is worth shopping around. The exception to this is Books on Board which sells MOBI ebooks from mobipocket.com and from OverDrive. So BoB can have an ebook in MOBI format that other ebook stores don't have.
In the UK and Europe there are many ePub-only ebooks. See
Adobe ADEPT DRM for EPUB circumvented, for how to remove the DRM. Calibre does a good job of converting DRM-free ePub to MOBI.
Finally,
http://www.ebookprice.info/ searches multiple stores and multiple formats. So you can see if an ebook is available in some other format. In my experience, a few older ebooks are Adobe PDF only but most recent non-technical ebooks are availbale in either MOBI or ePub or both.