German Swedish French - nationwide auto factor.
Or EuroCarParts.
It may be that the engine and gearbox have been settled too far back on the mounts after the transmission was out. Loosen the mounting bolts and hoick the unit forwards, then re-tighten the bolts and see if that makes any difference.
If it's only been doing it since a rebuild, I'd take it to the place that did the rebuild, if they installed it.
If you do replace the mounts, go 1.3 Golf back gearbox mount, standard front mount and Corrado G60 rear engine mount - nice tight set up.