The most common way of purchasing an aerial is to choose the aerial itself, then a suitable mount, then a connecting lead...
Most of the aerials featured here will simply screw into a "standard" CB aerial mount with a 3/8" UNF thread.
There are other types of mounting, these are...
DV mount which uses two halves of a circle with a wing nut and bolt which holds the two together, which also allows you to angle the aerial.
PL mount often found on the better quality aerials, this is a wider connection, and is usually supplied with a weather cap for when you want to remove the aerial.
Most of our gutter and mirror/rack/bar brackets can be fitted with either 3/8", DV or PL to suit the aerial.