Taz, I'm sure there have to be a bunch in LA County but I'm having the Viper-X done at Fuggin in Messa, AZ. One thing that made it doable was the fact that they had the header pipes already in production and the custom fabrication was only for the collector and muffler. Best thing to do is submit a Photoshop rendering of what you want and get a quote from somebody local unless you are not in a hurry since long distance vs. face to face slows the process.
This is my actual pipe 'under construction'.
This was my original rendering which was changed a bit to make production more feasible.