I have made 3 barrels for the panther in the past. You need a Pro-lite/A-5 threaded barrel, but it needs to be one with the smaller diameter base(the ones that slide into the foregrip on the pro-lite) old Armsons are great. old SP All Americans work to, and J&J still keeps it thin on their one piece.
What you do is file off the leading threads until the space between the tip and the start of the threads match the stock barrel. Then file the base of the barrel after the threads also matching it to the stock barrel.
Its easy to do in a lathe and then you could use any Pro-lit/A-5 barrel, but I have done all mine with a file.