I know EXACTLY what you're talking about. It should be somewhat easy to find a donor latch.
You also might try going to McMaster-Carr, and industrial supply house, and see if they've got a spring in their catalog that might just swap in. Spend some time in their online catalog... they've got EVERYTHING. Awesome place.
My problem with the rear barn doors on my '62 is that the catches are all so worn, there's no adjustment in them. That little latch you have to flip with your finger? On my truck, that jaw is worn a good 1/4-inch. The round rod on the other half of that latch is worn half way through on mine, for both the top and bottom catches. 47 years of daily driving has taken it's toll!
-Brad