Is there a cheaper alternative to aluminum wire for armatures?
A couple of alternatives. 

Firstly, cheap and cheerful, use metal clothes hangers (normally given free from the dry cleaners). However, this would only offer you a limited range of gauges.

Secondly, pop down to your local DIY/Garden Centre. They will have a large selection of fencing &amp; plant growth materials. You will find all sorts of wire used for fixing trellises, plant growth leads, etc.

Final alternative is to strip out the copper conductor from old electrical cable. 


  Make sure that the cable is disconnected from any electrical source


This will provide you with a selection of gauges depending on the conductor size. However, once you get above 4mm you find that the conductors aren't solid but in strands.

Of the above, the local DIY/Garden Centre will yield the best results.