Booster pumps can be used to increase your water pressure for your whole house or just your shower. The 2 main leading brands are SALAMANDER and STUART TURNER. Care must be taken when choosing the right pump for your water system. This link from the Stuart Turner website gives a description on which pump would be suitable to boost your shower.
If you do not have a cold water tank then your only option would be the pump in this youtube clip.
You also need to be aware not all showers are designed for high water pressure, therefore your shower may also need replacing if you install a high pressure booster pump.
You can also by power showers where the pump is in the shower unit, which means you have what looks like a large electric shower like the picture below. Installing this type of power shower on a stud wall is not advised due to the noise created when the pump is running.