Secondary Wire?

What's the current wisdom on secondary wire? We've got a 12.75" x 40" 
PVC form that needs rewinding (This old Tesla Coil - Pictures comming to 
the web site soon).

We removed 1100' of 18ga. stranded wire and now wish to replace it with 
around 2500' of new wire. We're on a hobbist's budget so go easy... 
Supplier suggestions would be welcome too... Thanks...