There are several "Night on the Town" or "One night Affair" type stores here in Orange County where you can rent gorgeous gowns for the night. I'm sure there are several in LA and close to your house.
I took my sister to the garment district in downtown LA for her prom dress and it was beautiful - we bargained with the lady and got it for one hundred bucks. I can't remember specifically where because there are several dress shops down there but I usually park around Pico and 5th but the alley is not where you will find the stuff. You have to go on the streets surrounding the alley. (Big banner... Santee Alley and it is really busy there.) As you drive around looking for parking, you'll see stores with dresses in the window. You want to go to the larger stores with colored dresses in the window since there are many that do quinceanera dresses which are usually white. I would shop north west of the alley. For example, if you know landmarks, walk up 5th or 7th toward Staple Center or any of the business buildings - don't walk toward San Pedro Street. http://www.fashiondistrict.org/page.aspx?id=125
Santa Ana has 4th street (It would help if you spoke Spanish there). This is 4th Street and Main. Park as soon as you find a spot and just walk down the street. There are tons of dress shops there too. Great prices.
Bridal shops have great prices on bridesmaid dresses and those can be very pretty.
At the mall, Charlotte Russe, H&M, Macy's, Forever 21, Windsor, Reference... will have inexpensive options for dresses. I don't know your price range but at those stores, you can find a really pretty dress between $50-$100.
If you choose the outlets (Cabazon, Elsinore, Carlsbad, Citadel...) you may find a good price on a designer dress but it will still cost more than you would pay for any of the above dresses without the label.
Good luck!