Leaf springs are relatively challenging to find in small sizes. But this is one of the fun elements of this hobby - finding your own answer to the problems at hand. If you have the ability to have a local shop oil quench some mild steel, it's easy enough to make your own half-round springs for a nominal price. I leave it to you to find out more. One clue I can offer is that horse-drawn buggy supply companies offer a menu of springs of all sizes...and purposes.
And finally there are modern version of half-round leaf springs that are made from carbon fiber.
|Air Trekkers CZ Carbon-Fiber Leaf Springs|
Below is the concept for the Type 69 axle, an all aluminum tube axle with kingpin joints derived from a square tube section of aluminum. It is strong, flexible, and will be easy to adjust. In hindsight, it could have been a square section which would have been easier to make square to the brackets. At the end though the round shape gives me the flexibility to change the caster angle at any time - an ongoing topic with cycle kartery.
The measurement for the axle was off the first time I cut it, because I forgot to factor in the length of the spindles. So i had to cut down the length to count for the very long Azusa spindles. To keep the front axle from shifting left-to-right, or twisting, there are two 'set screws' that lock the axle to this spacer collar, which frictions against the 'washer plate' on the top.
Choosing aluminum here saved me alot of weight. The steel axle as specified weighed 10 lbs without any hardware. It was strong, but not light. The aluminum version here weighs 4 lbs without hardware, but includes the kingpins and has a measure of flex.