For Remington barrels, I recently called Remington directly and they sent me to Cabela's (http://www.cabelas.com/) or Bass Pro Shops (http://www.basspro.com/).
Both had what I was looking for (20" parkerized w/ rifle sights, 2 choke tubes - rifled choke and IC choke). Cabela's was slightly less money ($142.99).
If you are interested, I do have my old 870 20" parkerized barrel w/rifle sights. It is IC only, with no choke tubes. E-mail if you are interested.