In the USA it is possible to transmit WIFI a television signal to a another television in another room of the house : one has to plug a transmitter in the first TV set and plug a receiver in the second TV set.

Are such transmitters and receivers available in Thailand?

They're known under many names. Some [Media Converter] actually packetize the A/V to send over Ethernet cable or WiFi (2.4 or 5GHz).

Other manufacturers just use the same license-free spectrum to convey the A/V signal (no WiFi involved, just the interference).

Wi-Fi to HDMI Video Wireless Extender with Audio

Video Sender

Wireless Video Extender

2.4GHZ Wireless Audio Video AV Transmitter Receiver Digital TV STB Extender

"WiFi Television" is a confusing term. People will confuse it with iptv (Television over Internet) something altogether different.

Also, most only send the Audio/Video and don't incorporate the IR Remote signal to allow Remote Control function to work from other rooms.

Have not seen them in any Thai retail stores.

Maybe Pantip.

Else, Amazon or eBay.