The only issue I have with using the DN2540 device is that it may be a little difficult to get in Australia, so I'll have to do some research.

Yes, maybe there's some forum member, who can help us to find a good source of high-voltage N-channel depletion MOSFETs? I have problems to get suited ones, too, from time to time.

As you did mention it, this current source doesn't work with enhancement types, because then there's no chance ever to get a current to flow via source drain path!

Good luck,
Kai