Winchester 1300 barrels will interchange freely with a 1200, and vice versa except for the 1300 Defender barrels, which only fit the 1300 Defender, due to the extended magazine moving the location of the lug on the barrel. I am told that if you use a 1300 barrel on a 1200, you should only shoot 2 3/4 inch shells, due to the ejector port on the 1200 receiver not being long enough to eject a 3 inch shell. Don't know from expereince, as I only have 1300's. You're best bet in looking for a barrel is ebay, many 1300 barrels come up on ebay, a fair number new in box in addition to used, and there are also a number of 1200 barrels that come up on ebay also, you just have to keep watching. You can also watch auction arms in addition to gunbroker and ebay.
Garry