Any regular airsoft inner barrel will do fine, but are expensive compared to a regular brass tube (both ways most probably requier mesuring and saw off at the right length). There are airsoft barrels that has a special teflon surface inside the barrel to make the bb travel with less friction.
If you just add the extension the original inner barrel is going to be to short wich may cause the bb to exit the inner barrel in a unstraight line. It may result in the bb colliding with the inside of the muzzel break and bounce of in some random direction. That has happend for me, but I will not keep the airsoft unit for now. Will upgrade later to recoil and scale barrel instead.