You can download apps for both the Amazon's kindle format - the same for Barnes & Noble.
If you want variety, there are plenty of Ereaders - Aldiko, FB Reader, Moon Reader to name just a few. Most handle the EPUB format. You just need to choose the one you like. There's EXPdf reader for that format. Aldiko and Adobe read it, too.
You can buy from Amazon, B&N, Googlebooks, Booksamillion. Just make sure you buy the format that fits the reader you choose. If you like free - there's project Gutenberg (public domain) Feedbooks or Manybooks which do public domain and Creative Commons. Tech writer and Sci Fi writer Cory Doctorow uses Creative Commons. A lot of new authors in all fields are really good.