Not sure if you still need this but I had my wedding dress made for me at a place on Av. Larco in Miraflores. I think it was called Belnovia, but I can't remember. It is on the second floor of a centro comercial (they have dresses in the window - you should be able to see it from the street). The centro comercial is across from the Parque Kennedy.
I got married here in Peru and looked at many places to have a dress made for me based off of photos I had. I probably went to 5 or 6 places to get quotes and talk with the seamstresses and this place seemed to be by far the friendliest and best. They were always on time with progress and appointments and I was very happy with my dress.