The Range Finder is described as:
Increases your accuracy and range
This can be found in northern Flor de Oro at Rebel Hideout Los Tramperos. The accessory case is inside the building and you’ll have to kick down the door at the front of the building to get inside. There’s no resistance here.
The range finder has a large impact in increasing your weapons range, and slightly increases accuracy, but largely reduces handling.
In all honesty I’m happy with the range capabilities of the ARs in the game, and if I want to take longer shots I use a sniper rifle. Unless you need to balance your AR stats because you’ve added other accessories which reduced your range then I wouldn’t bother equipping this. But it is easy to collect if you’re in the area anyway.