I thought that was only for the 2nd one? After that no more?Every time you enter a wilderness tile on Tarkus after taking the first synthsheath, there is about a 1% chance another synth sheath will spawn.
Given that Shekka is also supposed to get a synth-sheath expac eventually, I'm beginning to wonder why the balls we don't have enough sheathes for even half of the possible targets.