In addition to valve springs, one can also buy tool and die or even just get generic springs for this, which usually outright give you that value in addition to dimensions.
Pricing varies wildly of course but it doesn't really matter what the rate actually is so long as it's high enough that some length of partial (not full) compression gives you the required force AND you have sufficient space for the length needed.
Of note, a longer spring will give you much more consistent force due to less variation from fully expanded to fully contracted cells but again takes more space up - and isn't that important anyways really.
