For this Achievement to unlock you simply have to find an eye patch and loot a container with the eye patch equipped. Sounds easy enough, but...
There is no way to tell, where or what apparel item spawns when you enter a world.
Apparel items totally randomly spawn in backpacks and tool boxes (the colorful ones).
You can also find Apparel on bodies.
You really need to be patient with this one, as spawns are totally random.
Take your time and search every house, every car and every body.
Look out for Camping sites, there are usually lots of backpacks around those.
If you have the DLC, a huge one with lots of caravans can be found at Vrakudden Camping Ground, north east on the DLC Island.