Man, I've been hunting around for that kill knife now for a few hours, and I've got nothing.
Let's at least put together a list of qualities we know it has (or assume it has) so that we can all search more effectively:
-It looks to have a full tang
-Definitely a chisel-tip
-Blade length appears to be between 6 and 8 inches (My guess is 7 inches)
-It appears to be made of stamped or forged stainless steel (Milled or machined steel is possible if it's custom-made)
-Aluminum or stainless steel handle...I say aluminum.
In style it is close to a long boning knife, but has a tip and thickness similar to a hunting knife.
All that said, I actually wouldn't be surprised if this knife was the lone custom-job in the bunch. Remember, they have "stunt" versions of this knife that are collapsible to prevent injury to those unfortunate enough to play Dexter's victims.
The accident involving Jimmy Smits nearly stabbing a stuntman for real shows that they have a collapsible version, which is always easier to build from scratch than modify from a real knife. So if they want continuity between stunt and real knives a custom one would fit the bill nicely. It's not like it would be too hard to machine for someone with the right tools.
...Just a thought.
-Nick