The components of a good fence charger include an interruptor which is a component that turns the electricity to the coil on and off. It is what makes the fence have a sharp spike of voltage for about 5 milliseconds, then a 1 or 2 second rest period. Electric fences are required to have an interruptor so they do not trap animals on the fence. If the electricity was on all the time, an animal could get muscle spasms that prevent them from getting away. The interruptor prevents this. Modern chargers use thyristors or other components to keep the pulse to a very short interval, sometimes down to just a few microseconds. Voltage is also very specific and carefully regulated.
The coil is just a transformer that turns low voltage such as from a 12 volt battery into high voltage between 2,000 and 10,000 volts. Low voltage can’t shock under normal conditions. 10,000 volts on the other hand can arc across a large gap to shock whatever is near the fence.
A spike coil is just another coil that is triggered when the fence is shorted such as by a weed. The spike coil boosts the voltage and/or amperage enough to burn through the weed. A good coil design incorporates both the normal coil and spike windings in the same transformer. Instead of looking for a “spike coil”, just look for a fence charger that has “weed whacker” listed as a feature. Ability to burn weeds is a very common feature of fence chargers today.
edit: I meant to mention that some areas should not use weed whacker type chargers because they can cause grass fires under some conditions. They are useful here in the southeast, but would not be recommended in a dry climate.