Up until about 6 months ago, Ralph's in La Jolla would usually have some parsley root in stock. Whole Foods would have it on occasion, too.
I have not seen it at either place since then. I notice that Specialty Produce has it listed on occasion, but I do not know of any grocery stores that carry it. Are any of you aware of any stores that might have it? Obviously (to some), these will probably be stores frequented by Eastern Europeans.