I have that same door handle on my '55 Courier, except mine is a door rather than a tailgate. I took mine all apart in the late '90's to lubriplate everything, including the spring. Everything was in good shape so I didn't need to replace anything.
If you need a replacement spring, take yours out and take it to the hardware store and head for the fasteners dept. It's probable that you might find a suitable replacement. As I recall, everything was easily disassembled and re-assembled without the need for any special tools, but being more than 20 years since I last messed with mine, I don't remember fine details.