'Normal' channels such as magazine 'for sale' lists will give you a start, although I haven't seen much recently. 'Other' channels, or 'the grapevine', can be much more productive - that's how we got ours a couple of years ago. The magazines, we found, don't necessarily tell you the whole story of what might be for sale out there....So try calling the Chipmunk Club, or even DHSL.
I wonder which one was flying at Perth today.......