find out if its a UHF, HF, or 27Mhz CB. most are UHF, but it you get the wrong antenna, you'll get no signal. then basically search on ebay for a UHF car cb antenna,
get one like this?:
NEW Pyramid 27" Magnetic Mount CB Antenna Car - eBay Antennas, CB Radio, Radios CB, Ham Shortwave...