Hi Francesco,
A few years ago I would have told you to stop by the Philadelphia Zoo but the caudate collection has been severely reduced due to a different focus on the collection. They do have Cynops cyanurus (mislabeled as C. ensicauda), Pleurodeles waltl on exhibit and a couple of other species off exhibit (Salamandra salamandra, Tylototriton shanjing, Desmognathus quadromaculatus, and one of the Pachytriton sp). There are some other unusual amphibians off exhibit such as Schistometopum thomenses, Dermophis mexicana, Bufo baxteri and others. It depends on what you want to see. The Staten Island Zoo Reptile house is being renovated and the last time I was at the Bronx Zoo the exhibit collection was not caudate heavy.
I can vouch for Mike having a large number of species of caudates....
Ed