Andy:
The dash mounts (oval shaped sticky things) and the beanbags are common to all the Garmin aviation products as well, so if you have an airport nearby with a pilot shop, you can probably get the mounting bases and spare sticky pads, etc. there.
If you can't find a pilot shop, just look for an avionics company (aviation electronics company) at the airport, they probably stock all that stuff.
The SP26xx specific components are not used in any of the aviation products, but the SP III components (cables, etc.) are used in the GPS 295 aviation GPSR, which is the same form factor as a SP III.
When I need parts and I am on the road, I just head to an airport - never been disappointed yet.
PanEuropean