LeeAnne,
There is a store in the Valley called Dressed Up! It's in Tarzana. You will not find a better place for formal/semi formal wear. It's similar to going to the Garment District and the best prices on the nicest things. I got my Stepmother of The Bride and Stepmother of The Groom dresses there and a few other formal things that are incredible. You can spend $100 or $1000 there.