You can buy gearbox parts at:
http://www.axel-augustin.de/shop2/Fiat-Professional-/Fiat-Ducato-Typ-280/290-2-5-TD-bis-Bj-94/Geriebedichtsaetze-Getriebe-Getriebeteile-Tachowellen/Getriebeteile-5-Gang:::3_173_77355_77364.html
This company has very modest packaging costs.
The change of the fith gear parts is rather easy to do without removing the gearbox!
Success,
Bert