The place you are looking for is "Palacio de las Novias" (Brides Palace). It's an entire block made into a bridal mall, in which you'll find pretty much EVERYTHING
you need as a bride or for a wedding: dresses, gowns, cakes, corsages, bridal "spa", shoes, jewelry, photo and film services, invitations, flowers, lingerie for that special night, I even think there are travel agencies there, but I'm unsure.
Location: Corners of Lampa and Huallaga, in the Historic Centre of Lima.
Go in light clothing, as these days are getting sunnier. Also try not to carry any visible eye-catching jewelry and go with someone, even better if you go with your entire bridal entourage. They also organize runway shows, so brides can see lots of dresses. Good luck!