If you don't want to build a video modulator use one taken from old otherwise borken VCR ..
Those modulators are typically about matchbox size metal boxes with antenna connector, take in power (usually around 9VDC), video signal and audio ..
Output is RF suitable for TV ..
Signals from those modulators are rather weak and might only work if all TVs are connected to the same network ..
Otherwise you need to substantially amplify signal from the modulator by adding proper power RF amplifier ..
Eg. Ikusi - Line and Distribution CATV Amplifiers