The Jetta bodyshell was designed as a saloon version of the Golf floorpan – they didn’t take a Golf off the production line, chop off the back of it and then weld a boot on to it – it’s ‘bespoke’ (great mental image there though!). Because it shares Golf Syncro hard-points throughout the shell there is huge parts interchangeability.
I fitted Golf GTI seats to my Jetta Syncro. If I recall correctly the Jetta Syncro is the only Jetta you would be able to do that with. The 2WD Jetta had a fixed rear seat – not interchangeable.
Basically, any Golf seats will fit your Jetta Syncro, as DannyP has said.