To make my handrails, I use standard brass wire that I straighten myself. This process is not only simple, but is also a lot cheaper than buying brass rods.
Step 1: Cut desired length of wire (with a little extra).
Step 2: Put one end of the wire in a vise, and put the other end in the collets of a power drill.
Step 3: Pull the drill back gently while running it fairly slow. With larger gauge wire, this process may heat the wire excessevly. This process can take 10 to 15 seconds. Make sure to always wear eye safety equipment, as the wire could break.
Step 4: Trim the ends of the wire where the the vise and the drill made contact. These finished wires can be made at any length and for any modeling purpose. Please note that larger diameter (18 gauge) stays much stiffer than the smaller size (24 gauge).
With a little bit of practice, a wire like this can come from from the spool to perfectly strait in under a minute!