Where do you find the Ferelden locks?
Solution 1:
These drop from hostile mages in the Hinterlands. One wiki entry I've found states that they're found on mages near Dwarfson's Pass, but they should drop from mages anywhere in the zone.
However, you may have already killed everything (or nearly everything) in the Hinterlands. The only solution I've seen so far involves cheats or a duplication glitch.
See this reddit post for more information on that: Where do you get ferelden locks if you've killed everyone in the Hinterlands.
Solution 2:
It's a matter of timing, if you go after Wicked Eyes Wicked Hearts you'll have to farm it from the stragglers. This is the same for decorative gems. Once the red Templars move in the items are next to impossible to find.
Solution 3:
Templars can be farmed south and southeast of the Upper Lake Camp, between the lake and the ruins. The drop rate for the locks though is super low. Kill the Templars then travel back to the camp, then walk the area again and repeat. May occasionally need to travel a couple times to get to respawn. Be prepared to be there awhile. But you can get anywhere from 2-6 Templars at a go.
Solution 4:
After a partial playthrough and my initial playthrough I eventually found some.... I had not been able to find any at this stage in my previous playthroughs as the Hinterlands Cage Requisition was one of my first quests on my partial playthrough and I stopped playing that one and started again a I couldnt find any, on my second playthrough I had the requisition request as the second one, again I had plenty of enemies left in the Hinterlands at this stage but no locks... My video probably wont tell you anything you dont already know but here it is anyway, they do exist!!!