Try the GunParts catalog. They list parts for several 10 ga shotguns.. Also a trip to local pawn shops might find such a piece for sale. DixieGunWorks used to have a large selection of old shotgun barrels. Some were doubles and they were quite old.. Some of the barrel makers also sell 'rifled' shotgun barrels(not sure if in 10 ga.) and likely could make one without the rifling.. What are you building if I may ask??