Finding CB antennas "in the wild" these days is pretty much limited to truck stops and Amateur radio shops (although not all Ham shops want anything to do with CB, and others are quite happy playing both sides.)
Major truck stops (T/A, Petro, Flying J, Pilot, Sapp Brothers, etc.) will almost always have a selection of a few brands, makes and models.
Radio Shack still carries a few models, but most retail stores will have one or two at best, and most likely left over from the first Bush administration...and IMO Radio Shack's antennas are pure junk unless you're going for a 108" steel whip.
The base/attachment you're describing is pretty much an industry standard 3/8" fine-thread mount, so should be pretty easy to replace once you find an outlet.