Where can you buy a rifled barrel for Western Field M550ad?

Western Field was a Sears Roebuck & Co (Sears) brand name used by numerous manufacturers over the years depending on the time frame and model of the firearm.

The M550AD appears to be a model number associated with a bolt action carbine available in .22 Long Rifle, .22 WMR, and .30-30, the latter of which would indicate a rifled barrel.

Therefore, you would need to identify the manufacturer of your specific rifle and then likely order the barrel directly from them or through a distributor.

Some potential avenues are Numrich Gun Parts Corp (Brownells), Jack First Inc, or directly from the manufacturer if they are still in business.