Ordway in Van Nuys and McLogans in LA and Chatsworth (I think) are my guys in LA. Ordway has a great staff for vinyl stuff, McLogans has EVERY kind of sign gear and are cool folks too. Ordway also will do cutting and printing wholesale so if you need to use that $20,000 printer/cutter on a job, you can have them do it and not turn away work.
Ordway Sign Supply - The Complete Source for Today's SignMaker. is Ordway, Google McLogans, I don't seem to have it handy.