Wire armatures are usually made from steel wire or aluminum wire. The size or gauge varies, depending on how big your project is, and how much bending or wrapping you want to do. Steel can be hard or soft ('mild'). Sizes can be a bit tricky.  At the hardware store, they tend to label them by gauge (the larger the gauge number, the thinner the wire), but if you want to buy armature wire, they may label it by fraction of an inch, gauge, or mm. (They really need to let women take this measurement business over, and make some sense out of it!)

You can take a heavier wire, like coat hanger wire (which comes in at least two sizes), and just form a basic shape in the proportions you want. If you use several pieces, you could wrap the larger wire with smaller wire to hold it together. 

Generally speaking, the larger the project, the heavier the wire should be. I met one woman who uses that bare electric fence wire for keeping livestock in pastures for many of her armatures. You'll find it in farm stores or sometimes, hardware stores.

Some art/craft stores sell heavy aluminum wire that is really closer to being "rods", except that its usually sold in coils.  I've seen it 1/8" thick to 3/8" thick.

Some artists do a LOT of wire wrapping over wire.  A human body, for instance, would have the wrapping built up in the upper arms, less in the lower arms, more in the thighs, less in the lower legs. But it should never be too close to the actual size/thickness you want in the finished project.  Use the PM for that.

Once the figure is fairly steady, wrap it with a bit of crumpled or wrinkled aluminum foil where its to be thicker.  PM is rather difficult to stick directly to wire -- it wants to spin around a thin wire arm, for instance.  Add some foil "muscles" and the PM will stick much more easily.

Some people use plastic-coated wire, and sometimes the PM sticks more easily to that. But coated wire is often quite thin, so you might want to make "bones" of several thicknesses of wire, which are then wrapped with wire to make "muscles".

Always make sure that your armature is steady and firm before you go on to adding the papier mache. If it flops or twirls or falls apart, it will do the same while the PM is drying, and may not turn out as you anticipated.

If you intend to attach your project to a wooden base, you might want to think about drilling a hole or holes into the wood and cementing the ends of the wire armature into it before you start applying the PM, or leave the tips of the wire bare, so you can cement them in afterwards.

Make every attempt to bury or turn all the wire ends inward, or you will be cutting yourself with exposed tips.

If using steel wire, be sure to cover all wire with several layers of PM.  Any exposed wire will rust.

Its always nice to have a collection of different sizes and softness of wires handy, so you can use the size/kind you suddenly realized you needed.

Practice making a few armatures, and you'll catch on quite fast.

Sue