Dark Trousers can be exchanged for 200 Poe at Bargainer Statue which is located in locations like Lookout Landing. If it is not in the trading list, then unlock the Bargainer Statue in the Depths then it will be added to your list.
How To Get Dark Trousers Back
Available From The Bargainer Statue In Depths
Once the Dark Trousers is obtained, it will be available in the Bargainer Statue lineup in Depths, and can be reobtained in exchange for Poes. The Bargainer Statue lineup will be restocked during the Blood Moon, so as long as you have Poe, you can get it over and over again.