The short answer is that we don't know for sure. The Bands of Mourning are a set of magical artifacts that allow the wearer to access Allomantic and Feruchemical powers they otherwise would not have. They were created by the Lord Ruler, but it's possible that Kelsier had a hand in their creation as well.
We do know that Kelsier was an incredibly powerful Allomancer and Feruchemist, and he was also very knowledgeable about the Lord Ruler's secrets. It's possible that he was able to create the Bands of Mourning, or at least replicate them. However, there is no concrete evidence either way.
In the end, it doesn't really matter who created the Bands of Mourning. What matters is that they exist, and they can give people access to incredible power. If you're lucky enough to find a set, make sure you use them wisely!