Ariel's Grotto is your best bet since she is pretty much guaranteed to be there unless it is too cold or under refurb. If either of those are the case, she appears at Toontown Hall of Fame instead. Aside from the Grotto, the only place she appears regularly is the Storybook Princess Lunch and Dinner (not breafast usually). If she is there, she appears at the entrance and you meet her before you are seated. She is not guaranteed, but she is there pretty often. Here's a photo of her from when I saw her there:
I've also heard of her appearing at MGM when Voyage of the Little Mermaid is under rehab (which I don't think is scheduled for a rehab in the forseeable future).