We have a large range of trampoline springs, with accurate measuring you're likely to find a spring to fit your trampoline.
A Trampoline should have all springs attached and working, at the same tension, at all times. As soon as one spring is missing or over sprung it is recommended to stop using the trampoline until a replacement spring of the same size - length and diameter- is sourced and fitted.
A trampoline spring is measured by removing one that is still in tight spring condition, run the tape measure from "hook end to hook end" take the measurement in millimeters.
Also, measure the diameter of the spring. Sit the tape measure on the top of the spring and get a reading from outside of the coils you may need to factor in a millimeter or two if the coils taper off at the ends.
Best to buy more than one. Postage is almost always the same for a few.