Who the saiyajins got the scouters from?
The Scouters were originally invented by the Tsufurujins who used to originally inhabit planet Vegeta before the Saiyans took over the planet. However, the Freiza force found out about the scouters through the saiyans. Gichamu, a highly skilled engineer of the Freiza force found it and modified it for actual battle by implementing features that allowed telecommunication and concrete battle power numbers.Source 1Source 2