Pretty cool. Nothing says "vintage" like a leaf spring fork. Hats off for your efforts in bringing these to market. Other possible spring sources would be whoever makes them for golf carts and snowmobiles.
SB
You could also alter geometry so that the pivots were much closer tu each other, and the axle more distant from them, to dramaticly increase leverage against the spring.
I like your fork. Leaf springs do look cool, that's for sure. The longer the spring the more flex you will get. Here is a picture of mine so you can see the length.
My spring is off of a full size pick-up. So it's on the stiff side. My bike weights around 160 lbs and I am another 218 lbs. When I sit on the bike I can't see it dive. When I put a magnetic caliper gauge on it it showed 1/8 dive. It is a rough ride up to about 25mph then the loads begin to make the spring react and it's smooth. Try yours while going at speed and it will probably be comfortable for you. Too stiff is probably safer than too springy at speed.