Yep, Yuffie's available from any forest in the game - except for those on the northern continent. You can get her at any time in the game, though obviously she'll have more involvement in the story if you get her earlier. Same goes for Vincent, who's also a great character (not the strongest team member, but that can be changed with levelling, materia, equipment and so forth. Vincent's back-story is also very relevant to the game's main plot.