Is anyone aware of a resource that has curtains or other "interior decoration" type meshes? Or, if I had a modder's permission could I just take something from an Oblivion mod and run it through Blender to make it a Fallout 3 NIF instead?

You don't need anyone's permission if you're only planning on using it for personal use. Uploading for public use is a different story though. Even then it's simply a matter of respect, there are no real legal issues involved.