Rod stiffness determines the amont of rod whip. For a metal, steel is already pretty stiff, and only titanium is stiffer.
The longest 1/4" rod you are likely to find is 6'.
You can get them from
https://www.mcmaster.com
I would recommend 4140 chome-moly steel as it is far more reasonably priced than stainless and reasonably corrosion resistant and quite stiff. This material worked well for TARC egg lofter rockets in the 1.5 to 3 pound class.
8927K21 4140 Alloy Steel Rod 1/4" Diameter, 6' Length $ 3.74 Each
The following is a drill rod stock which is probably a bit stiffer but less corossion resistant and far more expensive. Not worth it.
88565K52 (Same as 88565K3) Grade D-2 Steel Precision Ground Rod 1/4" Diameter, 6' Length $ 26.84 Each
The stiffest material for launch rods is titanium. It is twic as stiff as steel, but it is very expensive, and really not worth it for a launch rod.
89055K33 (Same as 89055K53) Titanium Grade 5 Rod 1/4" Diameter, 6' Length $ 97.86 Each
Adding a joint will in all likely hood make a rod of the same length weaker.
You are much better off going to a larger diameter launch rod than by going to a more expensive material solution. While the relationships are dependent on a number of factors, a 3/8" rod (50% larger diameter increase) is usually more than a factor of 3 stiffer than a 1/4" rod. Obviously a rail is even stiffer.
Bob Krech