I used to live out in the Inland Empire and have a couple suggestions for you.
1st is a place in Fontana called the “CB Radio Shop.” It is on Valley Blvd, about Beech Ave. I have never been there but would assume they would definitely have CB antennas.
2nd is a place in Ontario. It is called the, “The Radio Shop,” and is close to the T/A Truck Center. I have been here and they do great work. This is where I go to have my radios tuned.
3rd There is always the truck stops right there T/A and Petro off of Milliken Ave and the 10.