Obviously, you can "cheat" using a Wondermail generator. If you want to work for it, there are a few ways:
Do pointless missions and check the boards every day. Eventually, there will be a mission with a reward of the item you seek.
Personally, I got a couple with a different method. Bring a trawl orb/warp scarf/pure seed/escape orb, and preferably a ghost type Pokemon or Darkrai, so you can walk through walls (but that's no issue, if you have Dusclops, I guess). Run Shimmering Hill or Mt. Mistral, as they have a high probability of having Kecleon shops with evolution items. Once you encounter a shop with a reaper cloth, rob that Kecleon blind!
If you aren't an experienced thief, the safest way is to use an escape orb or pure seed as soon as you grab his items. You can also use a trawl orb by the stairs and snatch up the items before you run to the next floor. Finally, for misers like me, you can take his stuff, use a warp scarf to get out of there, unequip it and go into the nearest wall. Since Kecleons can't walk through walls, you're safe until you get to the staircase room, where you'll have to somehow make it to the stairs before Kecleon can kill you.
Again, alternatively, although this is an essentially stupid method, if a friend of yours has a reaper cloth, simply suicide in a rescuable dungeon and have them rescue you. Then, they may give it to you as an attachment on your A-OK mail. That, or you can rescue them, and they can attach it on their Thank-you mail.
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you have to put a reaper cloth on duclops you have to put a reaper cloth on duclops you have to put a reaper cloth on duclops you have to put a reaper cloth on duclops
Dusclops uses a Reaper Cloth to evolve into Dusknoir when traded.
Reaper Cloths don't exist in Pokemon Sapphire.
Dusclops. Trade a Dusclops holding a Reaper Cloth to evolve it into Dusknoir.
Dusclops uses it to evolve into Dusknoir. Equip a Dusclops with the Reaper Cloth, and then trade it to initiate the evolution.