The saiyajins were a primitive race without technology. Freezer's army use scouters, so you could think saiyajins got the scouters from Freezer. But when Kaiosama tells Goku the story of the saiyajin, in a flashback, the tsufurujins when fighting the primitive saiyajins are seen using scouters. So who the saiyajins got the scouters from? Freezer/Freezer army, the tsufurujins or any other race?
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.