Here is one for sale from Andy, I would say if you made a reasonable offer you could get it for a good price:

http://datsun1200.com/modules/newbb/viewtopic.php?topic_id=39581&forum=2

Here is the link to the A15 & 5 Speed Roo is talking about.

http://datsun1200.com/modules/newbb/viewtopic.php?topic_id=39546&forum=2

The phone number is:

8369 1111 The prices are pretty steep, but he insists the A15 is a runner, so you just need to drop it in.

The 5 speed would be of little use unless you have the cash to pay for an adaptor kit from Japan, which costs a fortune in itself. You'd end up paying more for converting the Vanette 5 speed than you would buying a standard 5 speed gearbox.