I use sterling silver and it costs less then most material I have found. Now 14 and 18kt gold is expensive. The most expensive material is the stuff direct from DOVO. Stupid prices for a simple pin and washer. Silver looks great on ebony and ivory.
Silver is about $14 for a metre of 1.6mm rod. Actually it comes in a coil.
It is not the easiest material to peen however , it takes longer than soft copper/brass mixes.
Try searching under "jewellers fines". Check out the bead washers for pin washers.