Cabela's, Gander Mountain, Hastings and Brownells are some good places to buy replacement shotgun barrels. Most of the stores sell replacement barrels for all of the most popular brands, including Browning, Remmington, Mossberg and Benelli.
Cabela's has a wide selection of replacement shotgun barrels that can be purchased online or at one of the dozens of Cabela's stores located throughout The United States and Canada. Gander Mountain has a similar selection and shopping options with the bulk of its stores located in Texas and east of the Mississippi River.
Brownells has been selling firearm parts since 1939. The company offers shell-to-slug conversion barrels for shotguns, as well as standard replacements. Hastings has its own brand of replacement barrels made by Verney-Carron in addition to a large stock from other manufacturers.