If you don't have your heart set on a designer name, you might want to try "Bridal Alley", a stretch of Las Tunas Ave. in San Gabriel/Temple City. There are a ton of bridal shops there that sell gowns for less, but it takes some hunting. Most of them have pretty low-quality, no-name dresses, but a few have some designers (La Sposa, Demetrios, Maggie Sottero, etc.).
I recommend Fionna's Bridal on Las Tunas & Temple City Blvd. They sell and rent gowns, but if you bring in a photo of the style you like, they will make it for you, prices starting at $600. I ordered my gown with them and it should be ready for the first fitting in a couple of weeks.
Songi Bridal in Pasadena is also a very charming place. The owner and designer, Songi, is extremely nice and will meet you himself if you make an appointment. They will also custom make a gown for you, but are a little pricier, around $3000 for a custom gown, but if you order more than 6 months ahead I believe they'll give you a discount.