I assume that the spring set already has a full set of 9 springs. If you know the spring rate of the current springs then I suggest you go to a spring manufacturer's website such as "Associated Springs" and check out their catalog for "In Stock" extension springs to see if you can find one with a higher spring rate of similar closed length to your existing springs.
Note: When selecting a new spring be sure to check that its maximum allowed extension is equal or greater than the extension length of your springs when the bed is open. (What you will be looking for is a spring equal to your current ones but with a larger wire diameter.)