You can purchase wireless remote control devices that typically come in packages with one remote control, and some number of devices that you plug into an outlet, and then you plug whatever you want to control into those devices. You can easily find them on Amazon or eBay, or at big box stores or at places like Harbor Freight. On an online site, just search for “Wireless remote controlled outlet”. The main thing you want to make certain of is that they are controlled by radio frequencies, which most are, and not by an infrared signal like most TV remotes. ….. The idea here is that we want the Raspberry Pi to emulate the same signals that pressing buttons on the remote would do. This allows the Raspberry Pi to control the outlets and opens up possibilities limited only by your imagination.

Source: How to use a Raspberry Pi to trigger wireless remote controlled outlets (and probably other wireless devices) – Two “Sort Of” Tech Guys