http://www.e-gunparts.com/ordercatalog.asp
Numrich Arms/Gun Parts carries replacement barrels for the conversion.
I have a Ruger Ranch Rifle in 223 and it isn't very accurate, 3 inch groups @ 100 yards are rare. I suspect the 5.57 Johnson will be the same.
Costs: $450.00 for a doner M1 Carbine + parts looks too expensive. But then again, sometimes you just gotta......