Punto Azul is a good cevicheria a short walk from Parque Kennedy. There is also La Mar, owned by Gaston Acurio of Astrid & Gaston. On Av. La Mar, around the 1200 block I think, there are a bunch of ceviche restaurants.
There are plenty of hole in the wall spots all over the place; just pick one and go for it. Or better yet, pick a few.