Looking for portable FM transmitter & compatible receiver headsets

Hi, I'm new to this forum and to AV equipment.

Does anyone know where I can purchase portable FM Transmitters that sends radio signals (audio) to multiple FM portable receiver headsets (like portable radios)? Receiver headsets must allow for several fixed frequency (channel) selection, rather than just providing auto-frequency scans. I'm looking for 1 transmitter and around 20 portable receiver headsets.

I'm hoping to connect the transmitter to the output channel on our PA system, then to project sounds to specific "channel" on the receiver headsets.

I have looked on Ebay and found some cheap 4 channel FM Transmitters, but cannot find any FM portable radios that allows fixed frequency selections.

Have looked into Google, but most sites link to hearing aids or expensive theatre products that cost £1200 onwards.

Appreciate anyone who can help. Thanks.