In OS X 10.9 (Mavericks), at least, theyre not stored as individual text documents in a place where you can see them and manipulate them.
Even the notes that you choose to save On my Mac end up sandboxed in places like ~/Library/Containers/com.apple.Notes/Data/Library/CoreData/ExternalRecords/AA97DB8E-73B4-4C75-B54F-B39E5BC7521F/Note/_records/0/p6.notesexternalrecord, where ~/Library means the (usually invisible) Library folder that sits alongside your Documents, Music and Pictures folders.
You can see them by choosing Go to Folder
from the Go menu in the Finder and pasting in ~/Library/Containers/com.apple.Notes/Data/Library/CoreData/ExternalRecords. Youll find yourself looking at a folder with a 32-character hyphenated name, which you can open. Then open the Note folder and then the _records folder. All your notes are two levels further down.
Alternatively, use the following Terminal command:
open ~/Library/Containers/com.apple.Notes/Data/Library/CoreData/*/*/Note/_records
Theres not much that you can do with them, however: they are not text or .rtf files, and I suspect that moving any of these files around or changing their contents directly might very well render individual notes irrecoverable or possibly even break the Notes.app altogether.