I have a 1300 20 ga. I am still kicking myself for not buying a rifled barrel I found at a flea market. Anyway, auctions and flea markets & roadside Yard sales can yield results. Takes a little Patience. You also have to look at a lot of stuff only your wife would be interested in....that's the worst part!
Also, check out small out of the way gun shops. They often have spare shotgun (new & used) barrels for sale.