Power microphones.

Power microphone have a built in amplifier to make your voice louder.
There are many different types and they may range in price from less than £10, although some may cost more than some CBs do. You should be aware that some of the really cheap ones may make you sound worse than you did before, and may just cause you to interfere with other channels without any great improvement.

Another point to note is that if you use one in a vehicle, any noise in the vehicle will also get louder. In a noisy environment, you may gain nothing as the noise may cause problems for people trying to hear you.
For noisy situations, there are special noise cancelling microphones (see below).