You could start by looking on websites like Viz Media or Comixology. They often have a wide range of manga titles, including Nana. Sometimes, fan-made forums or communities might share links to where you can read it, but be careful as not all of these might be legal or of good quality.
The Nana anime wraps up earlier than the manga. The manga continues to unfold with more complex and in-depth storylines that weren't included in the anime adaptation. It's kind of a bummer for fans who wanted a complete visual adaptation.
The Nana anime doesn't exactly match up with a specific point in the manga. It kind of stops prematurely, leaving out some crucial plotlines from the manga.