At one point Barrelhead was owned by Dr. Pepper, but I am afraid that it is no longer produced. You might be able to find some old bottles on eBay, but sadly you won't find on the shelves of your local store.
Barrelhead Root Beer is a brand of root beer manufactured by Dr Pepper Snapple Group. It is marketed as "draft style" root beer. There is also a brand of barrelhead root beer called Barq's that is available in southern cities. Barq's was founded in 1890 in New Orleans and is owned now by the Coca Cola Company.
Barrelhead root beer is still available in specialty grocery stores where other unique kinds of regional soft drinks can be found. Look for barrelhead root beer in select stores in warmer climates or in high-end stores where customers can afford to pay extra for stock that moves slowly. Other kinds of root beer are ubiquitous.