Are you watching the Equipment Exchange forums? If anyone is selling, that's where you are likely to find them. And you can post a Want To Buy ad. Also look at Remington's website to see if they are making an accessory barrel for your 11-87, then you can ask a Remington dealer to order a new one for you.
My 870 came as a combination set with a rifled barrel. I don't use it much, but it is nice to have the adaptability in a shotgun.