I think that you won't find a belt that also acts as a whip, normally those things are attached to the character's geometry or even drawn on it to save up poly counts.
Depending on what you want to do, you can animate someone taking their belt off and whip someone with it.
If you do not need to show the character taking it off, it is as simple as modelling a belt, texture it and then add some bones so that you can bend it.
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If you use blender, just google for a few tutorials, modelling a simple belt should be easy enough once you learn how to use Blender.
It has also tutorials on animations, rigging, etc. If you invest some time into it, you will be able to make it yourself and have the knowledge to make other things you may need in future.