Breakers yards are companies who buy written off cars and strip them for parts. There will undoubtedly be a set of identical back seats out of a written off Evo kicking about. Phone round locally or there are some great websites where you can specify exactly the part you want.
I think that if anyone had any idea how to solve your problem, they'd have posted about it after one of the four times you've asked.
If you asked me to guess, I'd say that you've bent the metal on the back of the seats. It probably won't have lost strength, but will have stretched just a little bit meaning you are getting this "popping" between two different states.
There are probably a lot of ways to fix that, but they are also probably a lot harder than either getting some replacement seats or ignoring it.