Rugged Leather...
Lots of places. The best bets that come to mind are Dredge outside Sorrow’s Embrace/at the Commissar world boss spawn, and Flame Legion in Iron Marches, both the southwest and the north. Risen in southern Bloodtide Coast/northern Sparkfly Fen should be good too.
Basically, you’re hunting level 50-60 humanoid enemies and killing them for salvage and bag drops. You’ll be able to pick up a good amount of Linen as well, and if you’re doing the Dredge by the Commissar you can take out that world boss for a chance at the 150ish gold Commissar’s Manifesto along the way.
A Commissar’s Manifesto can buy you a lot of Rugged Leather.