Adam, I've collected a few over the years in Australia but they are not common. One gunsmith here - who is mainly a barrel maker had some of the receiver components (which are the part of the barrel the bolt locks into) and could make up barrels. I'm not sure what the situation is now.

The 66 is a great rifle - 2-3" shorter than a standard bolt action because of the telescoping bolt and very accurate - possibly because it is functionally pillar bedded and free floating. Remove two screws to swap barrels, another screw allows you to swap trigger groups in seconds (eg from double set to a single stage - useful if going from 243 to 458.

mike