I am not aware of any aftermarket barrel maker offering them.
I think the only way to get an extended barrel for any model KelTek would be to have a good gunsmith make one.
Perhaps a cheaper alternative method would be to counter-bore the muzzle of a stock barrel and make a threaded adapter sleeve that could be silver-brazed in place. (The threaded section could not be larger then the barrel O.D. or you couldn't get the barrel in or out of the slide.)
Only downside to that is there isn't much barrel wall thickness to work with on the .380.
It could be easily done, and I think it would probably be safe at .380 pressure levels, but I'm just guessing.
Proceed at your own risk!
rcmodel