Look at: System.IO.IsolatedStorage
You can manage your files using different scopes of isolation and don't bother about their actual place:
Application - Isolated storage scoped to the application.
Assembly - Isolated storage scoped to the identity of the assembly.
Domain - Isolated storage scoped to the application domain identity.
Machine - Isolated storage scoped to the machine.
None - No isolated storage usage.
Roaming - The isolated store can be placed in a location on the file
system that might roam (if roaming user data is enabled on the
underlying operating system).
User - Isolated storage scoped by user identity.
(from here: http://msdn.microsoft.com/en-us/library/system.io.isolatedstorage.isolatedstoragescope(v=vs.110).aspx)
But possibly you don't need such a control over your files isolation. In this case you can simply use local user's aplication folder:
string path = Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData);
Look at: Environment.SpecialFolder - a lot of useful places there.
And, of course, you can always use you application's folder:
AppDomain.CurrentDomain.BaseDirectory
Another ways of accessing it: Best way to get application folder path