Hi Julie,
I am in Southern California and like the others, high end garments rarely come my way and like you, I usually haven't a clue about pricing and such.
What I can tell you is that you will most likely find your source on Melrose, or in close proximity in Los Angeles. There are a few in Venice also, but I'm not sure of the names. Maybe a search will help you.
Because we live in close quarters with the Hollywood set, your best bet may be to contact a few of these shops and see if consignment is an option.
My favorite place is called "Aaardvark's" in Pasadena. They have wonderful vintage garments. I used to take my stepdaughter there for her Halloween costumes and I've bought a few pieces there. Not sure if they consign, but it can't hurt to call and find out. Maybe the call will lead elsewhere. Here is their address below:
1253 E Colorado Blvd
Pasadena, CA 91106-1834
Phone: (626) 583-9109
Again, I cannot vouch for these two stores, I've just heard their names mentioned around the vintage clothing stores locally, but maybe if you call them you can see if they do consignments. I have found that most of the other vintage stores are just fluff for the Hollywood types that are not as interested in the designers, as they are the trends of the day.
Decades
8214 Melrose Ave
Los Angeles, CA 90046-6812
Phone: (323) 655-1960
Lily et Cie
9044 Burton Way
Beverly Hills, CA 90211-1661
Phone: (310) 724-5757
Hope this helps you and good luck!
Nadine