Armature wire is used inside of cloth characters to give them a “skeleton” so they can hold a pose. The skeleton is optional.
Thick armature wire is 1/8″ or 10 gauge and I use this for the legs, body, and necks of my cloth characters. This brand on Amazon works just as well as the similarly priced version I find in Micheal’s Crafts in the clay section.
Be sure you have a decent pair of wire cutters and needle-nosed pliers to work with any wire armatures.