Lee Spring is the most comprehensive supplier of springs that I know and they don't have one that would meet your requirements. From the print, the spring rate is 5.14 lbs/in.
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You state that your spring is getting tired? By what measure do you come to that conclusion? Have you measured the free length of the spring or the spring constant? In my experience, springs usually fail by breaking. It is possible that you could revitalize the spring by stretching it. Your goal is to have a force of 18 lbs. on the spring when compressed to 6.5 inches. It would be fairly simple to make a jig to do this measurement. A disk with a slip fit hole for a guide rod to prevent the spring from going sideways when the spring is compressed and a bathroom scale. Mark the back end of the rod for the 6.5" length.