As far as I know, there are thousands of these barrels around, and I seriously doubt Remington has stopped making them. (Ah yes, the old "Since I don't have one of THOSE for sale, but I DO have one of THESE, 'They' have stopped making THOSE now", trick) I think Brownell's may have some, but I'm willing to bet you can find one quick at a very good price on the EE right here.
Of course, Remington, like most manufacturers does make "runs" of various products such as barrels, or finishes (they used to make only a single small run per year of parkerized 870 police models, for instance, so for a while after these guns hit the dealers, you would see them everywhere, and once they were sold out, they were scarce until the next run was made) To my knowledge, the vast majority of the 18" barrels will be cylinder bore only.