This cheap pair of radio modules (transmitter FS1000A and receiver XY-MK-5V) can be bought lots of places online, and all the hard work of using them (at least for Arduino) has been provided in the RadioHead library.
I connect a couple of Arduinos (Mega and Nano) and transmit a message in the 433MHz ISM band.
RadioHead library: http://www.airspayce.com/mikem/arduino/RadioHead/
My video on serial communciation: https://youtu.be/MYoMM1FY6yM
The source code featured in the video is on my github page: https://github.com/magiccow/ook