Hey Dave-
I had a hard time finding a cover, too, so when I bought the system to put in my car, I made sure it came with that piece first. You might try
www.car-part.com though. I can only say good things about that site. Very reasonable prices (because its a salvage site on which prices are actually posted on line), and its a searchable database that a tons of salage yards post parts on. I've bought a lot off that site for my cars and never had a problem.
Good luck!