Gun Parts had the 6.5 barrels for the Ljungmann, but they show them sold out and list nothing for the HAKIM. You might try Sarco, but I don't know of any large quantity of spare barrels for the HAKIM.
I have not looked at a HAKIM with a view to rebarrelling, but suspect it might be a pretty big (and very expensive) job to adapt a commercial barrel to that gun.
Jim