I wouldn't recommend getting Kindle for digital manga. If I'm not mistaken, all Kindle models have very little built-in storage - I seem to remember something about the devices being able to hold... 1000-2000 ebooks (not sure). That might seem like a lot, but it was meant for normal ebooks (text), not digital manga. So I'm guessing you won't be able to load that much manga in there. Unless you plan to frequently recycle your library.
Also, I'm not sure how well will the images present on KIndle screens, but since Kindles are black&white only, you'll lose colors on covers, the first few pages in a volume, and on full-color series.
Might be better to get a normal tablet instead. Or just read on pc/laptop or a smartphone, if yours' got big enough screen. But yeah, that's just my opinion.
As for where to get digital manga, well... the publishers usually put links to their digital manga on their websites. You can also just google "digital manga" or "manga ebooks". Two examples of results I got by googling these: kobobooks.com emanga.com. Hope that helps :P
