They are different heights to account for the extra weight of the FJ80 gas tank. The gas tank side is heavier...that is why it has a slightly longer spring.
As far as sourcing the springs, keep an eye on ih8mud.com that is where I got both sets of mine.
http://forum.ih8mud.com/sale-parts/2...m-springs.html
http://forum.ih8mud.com/sale-parts/2...prings-fs.html