Hmmm.... I'm not sure what size spring to get from a hardware store. Like BigArm said, I had salvaged the spring from a
Unistrut Spring Nut like the P1006 thru P1010 one. It seemed to be a good length and provided the proper spring compression to lift the trunk just enough to clear the latch pin. My local Home Depot sometimes carries those exact spring nuts and yours might also, if you wanted to look at them.
If you happen to have a friend, relative or aquaintence who is an electrician or other tradesman, they could probably just give you one. Many times, electricians just pull the springs off of the spring nuts and throw them out as they can sometimes get in the way when you are trying to put the stuff together and adjust it.