I know you're looking for straight wire, but coiled brass wire is readily available and inexpensive. You can straighten coiled wire by cutting a piece, gently running through your fingers to take most of the curve out, clamping one end in a vice , grasping the other end with pliers, and pulling hard enough to stretch the wire.  This straightens and work hardens the wire.
.016 = 26 ga. = 0.4 mm wire and is readily available in craft-type hobby stores or online.

But I'd suggest using the phosphor bronze wire for freestanding handrails. The material is stiffer, stronger, and springier, less likely to be damaged in service. If you're having trouble bending, you can anneal the spot to be bent. 
Jack Mullen