Echo Shell in Horizon Zero Dawn is a common resource, that is used for crafting ammunition (arrows/charges) or 2 Carry Capacity upgrades.
You can find the Echo Shells as loot from the Behemoth, Longleg, Rockbreaker, Stalker, Stormbird, and Thunderjaw. However, most of these machines are hard to defeat, so it may be a bit challenging to farm Echo Shells.
Another way to get Echo Shells in Horizon Zero Dawn is to buy them. Merchants will sell 1 Echi Shell for 50 metal shards.
Best Ways to Farm Echo Shells
You can loot Echo Shells from Behemoth, Longleg, Rockbreaker, Stalker, Stormbird, and Thunderjaw. The locations of these machines can be easily found by searching the map in the game.
However, I highly recommend farming Echo Shells from the Longlegs, as they are the easiest machines from the list above. They are not very dangerous and may not be hard to kill. You can find the locations of Longlegs to farm on this map.
Better to farm the Echo Shells in the locations on the screen below.
Those 2 locations are close to each other and both have campfires near. You can farm one spot, then go to the second, and when you are done, teleport to the campfire near the first spot, and the longlegs will be alive again.
Moreover, the are lots of bushes where you can hide and use your Silent Strike to kill Longlegs faster, without any risks.
Tip to Get More Echo Shells
I recommend to take the “Gatherer” skill in the early game, which allows you to find more Echo Shells or other resources from machines. It will help you always have the required amount of materials.
Buy Echo Shells from Merchants
You can buy Echo Shells from merchants. Since it’s a resource of common rarity, you can buy it from most wandering merchants, as well as merchants in cities.
However, for one Echo Shell you need to pay 50 metal shards. So you’ll need a lot of metal shards to buy them if you need to create arrows or charges. In this way buying Echo Shells from merchants is not very profitable.
However, if you need some Echo Shells to upgrade your Carry Capacity, it is not a bad idea to spend some metal shards and to buy Echo Shells.