Aside from physical copies still available on Amazon, is there any way to purchase this game? I'm looking for a (legal) digital copy, but haven't found it available anywhere online.
What happened to this game? It's gone from Fantasy Flight's site and DriveThruRPG
Odds are the licensing agreement ran out. Without a license in place, FFG (and by extension DriveThruRPG) can no longer legally sell copies of the game, especially not PDFs.
The stock that Amazon has probably isn't covered under the above, as any money that FFG would have earned has already changed hands, much the same as buying an out-of-print book from your local gaming shop.
It's back up; James Wallis got the rights back. PDF is once again available (and very slightly updated to reflect the new ownership):
Hopefully the print edition will be back out soon too!