As far as I know, you can't obtain the grim reaper costume without downloading a mod for your game.
As far as downloads go, there's this one: http://llnw.thesims3.com/assetDetail.html?assetId=988355
If you just want any sort of non-exact grim reaper hood, there's this: http://www.modthesims.info/download.php?t=368590
There's also a set of gloves here: http://www.modthesims.info/download.php?t=350010
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it depends what version of sims you have. On the Sims the grim reaper does not appear. On the Sims 2 you will have to sacrifice one of your sims to make the Grim Reaper appear. The sims 3 is the most fun game though. There are a number of ways to make the Grim Reaper appear, and you can even add him to your Active Family! Have fun! x
you die and the grim will show up at your house or it will be by your grave
Well when he arrives you can try burning him when he is near flamable objects
Talk to people, you can talk to almost everyone, even the grim reaper!
I've seen some people have Grim-Reaper babies from The Sims 3 on YouTube but I think there is a patch or something that stops you now, I could be wrong. See related link.