If you have an accurate bathroom scale (digital scale is better) then you can measure it yourself.
1) Remove the spring from the forks (obviously).
2) Get a rod which will fit inside the spring, and which is longer than the uncompressed spring. Any material will work, I used a piece of pvc pipe.
3) Stand the spring on the workbench, and slide the rod inside the spring till it stands on the workbench too. Now put a pencil mark on the rod which lines up with the top of the spring.
4) Remove the rod, and use a ruler to put a second mark some known distance from the first mark. 100 mm works well, but 50 mm will be easier to compress.
5) Slide the rod inside the spring, and stand them up on the bathroom scale. Now compress the spring (don't touch the rod) until it lines up with your 2-nd mark. Read the scale.
6) Now you know how many pounds it takes to compress your spring a known distance. Multiply that number by 0.45359237 to convert it to kilograms, and then divide by the distance between marks. For example, if your scale showed 39 kg, and your marks are 100mm apart, then your spring rate is 39kg / 100mm = 0.39 kg/mm