The upgrade toolkits are available for Dynamics Partners at Microsoft Partnersource. But since it's often very difficult to find your way around Partnersource, then it's also possible to download most of the upgrade toolkits from the Dynamics User Groups Download section.
If you're missing a specific upgrade toolkit then please request it in the forums or write a comment here.
Upgrade from Navision 3.70, Navision 4.0 and Navision 5.0.
The above link is only accessible if you have access to Microsoft Partner Source (US) - but then you find all the upgrade toolkits for the North American versions of Navision - from Navision Financials 1.0 to Dynamics NAV 2009.