Most CSLs in the UK stay complete with their kits... in the US, some do sell their barrel kits separately than the CSL. They tend to be be snapped up very quickly
So your options are
1, Buy a CSL
2. Look for someone in the States willing to sell theirs
They are nice kits, but honestly spend your money on better paint and use the stock barrel. They command such a premium that is not going to add anything to your game!