Where can you get a 30 inch full choke barrel for a Winchester 1300 12 gauge?

You should be able to go to Winchester cause my dad has one for waterfowl and it came with the receiver. Yiu may want to emphasize a 3 inch chamber for more likely match and due to higher pressures required for non-lead pellets

Answeryou could look on eBay or a wenchester department store
You should be able to get it in almost any gun shop.